There are a lot of precedent cases like yours.

people made various choice according to their own circumstance. This is not your personal decision.

1.You must consult with your son's mother. If she is doing well, money is not a big concern for her, she must consider her hu*****and feelings and the best interest for the boy.

2. sound like your current wife is not an obstacle. She already agreed with you.

3. If your son's mother said No to you, don't feel dissapointed. Blood bond is always there. When your son grow up, he can make his own choice. A lot of adult children will feel grateful to have a bio-father who really cares about him and a step-father who raised him and love him.

4. You can consider about "participation" if the ex-GF's marriage fell apart for the other reasons, such as domestic violence. Then, if the mother doesn't want to take care of her son, or has financial trouble to take care of him, you can offer to help.

only my two cents.
You need to make your choice, not only from your own emotional need, but also consider the best interests for all the parties involved.