My DH didn't meet his real father til he was 19.
His mother had lived in the same house for 10 years that he visited her in, then moved once.
His step father was scared it was the end of their relationship, but all was OK.
For a long time DH felt rejected coz he thought his own father didn't want him. When he met him & his father said that he tried to find him I kinda knew it was bull, but let it go for DH's sake.
I was 6 months pg with DD1 when we met him.
He had our number, & MIL's number (which has been the same for 16 years).
We rang him DD1's first christmas to let him know she was here. She was 8 months old. We haven't heard from him since.
DH got on with his life, but I know now he's a bit lost. He has a brother & sister to his Father who he would like to know, but after not hearing from his father in 5 years, he's not so intrested in knowing him.

From what I've seen, it might be best for your DS to make up his own mind. Maybe gently talk to him & suggest that he doesn't worry about that for now. That way when & if it does happen it'll just be a nice suprise.
It is harsh, but his fathers world obviously didn't stop when you found him, so your son needs to get on with his life too.

I don't know, maybe I'm wrong, but your poor boy is having his heart broken everytime he gets let down by him.
I hope you can resolve it before it has too much of an impact on his life.