Should a Buddhist pay homage/bend to his parents? I once heard we should pay homage (bending, prostrating or some similar) to our parents on their birthday. If that is correct, for who else should we pay homage/bend to? Monks and parents only?
From a Therevada perspective, in the Sigalovada Sutta (Pali Canon) the Buddha says to Silaga that parents should be worshiped. He doesn't actually specify monks in this sutra as at this point the Buddhist monastic community wouldn't have taken shape. However he does specify teachers and also ascetics and brahmans so we can maybe take this as the nearest equivalent.

In total he advocates the worship of 6 groups of people corresponding to the 6 directions so 


Parents as the East  
Teachers as the South 
Wife and children as the West 
Friends and associates as the North
Servants and employees as the Nadir 
Ascetics and brahmans as the Zenith


I have to say though he doesn't restrict the homage to their birthdays only. Sigala is portrayed as doing a daily practice of homage in the six directions to which the Buddha is responding. 

From a Mahayana perpective, the Ullambana sutra the Buddha advocates paying homage to parents   


  Every year, on the fifteenth day of the seventh month


so this would be at least annual homage which is nearer to your question. In addition there is also the Sutra of Filial Piety which goes on at length about the trouble your parents have gone to to raise you (which is fair I guess - they have gone to a lot of effort). This is probably as influenced by Confucianism as much as Buddhism. 

There are more of these type of sutras in Mahayana traditions but I'm already past my level of expertise so I'll leave those for other people to detail.