If he repeats ‘Umrah but he does not have any hair to shave, what should he do?
Question

He has performed an Umrah and wore his normal cloth but he did not shave his head. As he does not have hair (he has performed an Umrah two days before this). Then he asked a sheikh who advised him that, and told him that this is his exiting of ihram. Is that ruling right or he has to expiate?.
Praise be to Allah.If a person does ‘Umrah, shaves his head and exits ihram, then he does ‘Umrah again shortly after that, and his hair has not yet grown, then he does not have to pass the razor over his head, rather once he has finished his saa’i he may exit ihram and he does not have to do anything. But if some hair has grown, even a little, then he should pass the razor over it. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: It is better for the pilgrim who is doing ‘umrah to cut his hair and then shave it on the day of Eid. 
Questioner: On the day of Eid there will not be anything. 
Shaykh: But it will inevitably grow; the hair can reappear within a day and a night, and it is sufficient to pass the razor over that. 
End quote from Sharh Kitaab al-Hajj min Saheeh al-Bukhaari. 
And he said concerning the bald man who has no hair at all: He [Ibn Qudaamah] also mentioned in this chapter that if a person is bald and has no hair on his head, then shaving and cutting the hair are waived for him as they cannot be done; this is like the case of a man whose hand is cut off – he does not have to wash the upper arm instead. But he said it is mustahabb for him to pass the razor over his head. This is subject to further discussion, unless he is afraid that there may be a few hairs on this bald man’s head, so in order to be on the safe side (he should do that). This is how the hadeeth of Ibn ‘Umar may be interpreted if it is proven to be sound. But if he is certain that there are no hairs then passing the razor over his head is a waste of time. This is like what some of the scholars say about the one who is mute, if he wants to read he may move his tongue and his lips but there is no sense in doing so. The correct view is that if he is bald then it is not prescribed for him to pass the razor over his head because there is no point in doing so and it is a waste of time, and Islam does not prescribe anything that is a waste of time. But if he fears that there may be a few hairs there, then it is better to do it so as to be on the safe side. End quote from Sharh Kitaab al-Kaafi by Ibn Qudaamah. 
Based on that, if the hair had not grown back for the man asked about here,  because he shaved his head two days earlier, then he does not have to do anything and his ‘Umrah is valid. But if a little hair had grown then he should have passed the razor over his head. If he did not do that, then he is still in ihram and he must now take off his regular clothing (and put ihram on) and shave his head or cut his hair. Thus he will exit his ihram, and he does not have to do anything because of any forbidden things he may have done because he was unaware that he had not exited ihram for ‘Umrah. 
And Allah knows best.