I'm not on the birth certificate for my son, my ex partner as now moved on and in a strong relationship. The new partner would like to adopt my child can this be done? Due to circumstances I don't see much of him, so my son does look at him as his father.

DNA would have to prove paternity. Both parents would have to sign a consent for the Adoption. Many States require that the adopting parents be married and stable for (minimally) a year.

Generally, a court would require that the bio father be contacted to get his permission for a step parent adoption.

Sounds to me like you are willing to agree to this. So I would inform the mother that you are willing.

If you are the father, and have no objection to the new husband adopting, they will have to request the adoption and you, as the father of the child will have to sign over your rights, most likely DNA testing will need to be done to prove paternity