No reason at all why he should forgive - if he wants to continue to nurse his bitterness (which is quite understandable, don't get me wrong) then it's up to him.
However he should perhaps pause and consider that by cold-shouldering his real Mum he is in a sense replicating her original behaviour in abandoning him.
Having said that, it also depends on the real Mum's motives - are they selfish or altruistic - it would make a difference to me I'm sure. A lot depends on the circumstances, and condemning the lad is very presumptuous (and somewhat smug and self-righteous)