Was the Buddha a bodhisattva (for the benefit of all) before his awakening? Was the Buddha a bodhisattva before his awakening? Are there references from the suttas which point to when (possibly in past lives)?


  Bodhisattva is the Sanskrit term for anyone who has generated
  Bodhicitta, a spontaneous wish and compassionate mind to attain
  Buddhahood for the benefit of all sentient beings.


The following appears to suggest that the four immeasurables may have been present in the Buddha's past lives:


  The early Buddhist texts assert that pre-Buddha ancient Indian sages
  who taught these [four immeasurables] were earlier
  incarnations of the Buddha.


I apologize if the answer is obvious, but I haven't found a direct answer from the suttas.
Absolutely he was! In fact, there is no way to become a Buddha without becoming a Bodhisattva first. Numerous Mahayana Sutras attest to this. Without developing the immeasurable compassion of a Bodhisattva there is no way for a sentient being to become a Buddha.

As for the Theravada who do not regard the Mahayana Sutras as authentic Buddhavacana: the Pali canon still has many references to the past lives of the Buddha where he was a Bodhisattva. And they are not all part of the so-called Jataka tales either. See here for many examples.

You can find an example here - AN 5.196 - which is not one of the Jataka tales:


  “When the Tathagata—worthy &amp; rightly self-awakened—was still just an
  unawakened bodhisatta, five great dreams appeared to him. Which five?
  
  “When the Tathagata—worthy &amp; rightly self-awakened—was still just an
  unawakened bodhisatta, this great earth was his great bed. The
  Himalayas, king of mountains, was his pillow. His left hand rested in
  the eastern sea, his right hand in the western sea, and both feet in
  the southern sea. When the Tathagata—worthy &amp; rightly
  self-awakened—was still just an unawakened bodhisatta, this was the
  first great dream that appeared to him.


Emphasis mine.

And here is the original Pali:


  “Tathāgatassa, bhikkhave, arahato sammāsambuddhassa pubbeva sambodhā
  anabhisambuddhassa bodhisattasseva sato pañca mahāsupinā pāturahesuṃ.
  Katame pañca? Tathāgatassa, bhikkhave, arahato sammāsambuddhassa
  pubbeva sambodhā anabhisambuddhassa bodhisattasseva sato ayaṃ
  mahāpathavī mahāsayanaṃ ahosi, himavā pabbatarājā bibbohanaṃ ahosi,
  puratthime samudde vāmo hattho ohito ahosi, pacchime samudde dakkhiṇo
  hattho ohito ahosi, dakkhiṇe samudde ubho pādā ohitā ahesuṃ.
  Tathāgatassa, bhikkhave, arahato sammāsambuddhassa pubbeva sambodhā
  anabhisambuddhassa bodhisattasseva sato ayaṃ paṭhamo mahāsupino
  pāturahosi.


Emphasis mine again.

Here is another example from the Pali-canon which is not one of the Jataka tales:


  “So it is, Ananda. So it is. Even I myself, before my Awakening, when
  I was still an unawakened Bodhisatta, thought: ‘Renunciation is good.
  Seclusion is good.’ But my heart didn’t leap up at renunciation,
  didn’t grow confident, steadfast, or firm, seeing it as peace. The
  thought occurred to me: ‘What is the cause, what is the reason, why my
  heart doesn’t leap up at renunciation, doesn’t grow confident,
  steadfast, or firm, seeing it as peace?’


Emphasis mine.

And here is the original Pali:


  “Evametaṃ, ānanda, evametaṃ, ānanda. Mayhampi kho, ānanda, pubbeva
  sambodhā anabhisambuddhassa bodhisattasseva sato etadahosi: ‘sādhu
  nekkhammaṃ, sādhu paviveko’ti. Tassa mayhaṃ, ānanda, nekkhamme cittaṃ
  na pakkhandati nappasīdati na santiṭṭhati na vimuccati etaṃ santanti
  passato. Tassa mayhaṃ, ānanda, etadahosi: ‘ko nu kho hetu ko paccayo,
  yena me nekkhamme cittaṃ na pakkhandati nappasīdati na santiṭṭhati na
  vimuccati etaṃ santanti passato’?


And here is yet another one:


  “Bhikkhus, before my enlightenment, while I was still a bodhisatta,
  not yet fully enlightened, it occurred to me: “What now is feeling?
  What is the origin of feeling? What is the way leading to the
  origination of feeling? What is the cessation of feeling? What is the
  way leading to the cessation of feeling? What is the gratification in
  feeling? What is the danger? What is the escape?”


Here is another one:


  ‘Both the quotations you have made, O king, are accurate. But when he
  spoke of Āḷāra Kālāma as his teacher, that was a statement made with
  reference to the fact of his having been his teacher while he (Gotama)
  was still a Bodisat and before he had attained to insight and to
  Buddhahood; and there were five such teachers, O king, under whose
  tuition the Bodisat spent his time in various places—his teachers when
  he was still a Bodisat, before he had attained to insight and to
  Buddhahood. And who were these five?


And another:


  At Sāvatthī. “Mendicants, before my awakening—when I was still
  unawakened but intent on awakening—I thought: ‘What’s the path and
  practice for developing the bases of psychic power?’


Now, why did I emphasize "intent on awakening?" Because, this translation chose to use "intent on awakening" as a substitute for the original Pali which has this: 


  Sāvatthinidānaṃ. “Pubbeva me, bhikkhave, sambodhā anabhisambuddhassa
  bodhisattasseva sato etadahosi: ‘ko nu kho maggo, kā paṭipadā
  iddhipādabhāvanāyā’ti?


So I hope you can clearly see that according to both the Mahayana and the Theravada that the Buddha was an unenlightened Bodhisattva before he became the Buddha.

Now, was the Buddha as a Bodhisattva for the benefit of all? Here is a passage from a canonical Pali sutta describing the Bodhisattva Gotama being born into the world for the benefit of all:


  “The Bodhisatta, the foremost jewel, unequaled, has been born for
  welfare &amp; happiness in the human world, in a town in the Sakyan
  countryside, Lumbini. That’s why we’re contented, so wildly elated.
  He, the highest of all beings, the ultimate person, a bull among men,
  highest of all people, will set turning the Wheel [of Dhamma] in the
  forest named after the seers, like a strong, roaring lion, the
  conqueror of beasts.”


And from that same sutta here is the testament to his motivation:


  He, seeing the utmost purity, will set rolling the Wheel of Dhamma
  through sympathy for the welfare of many.


So, to sum up: have no doubt The Buddha was a Bodhisattva who manifested the Mind of Enlightenment - pure Bodhicitta - for the benefit of all before his awakening.