I have a 6 yr old handi capped child who's dad hadn't helped not seen him since he was 18 months old. I am getting married and my son want his "dads" last name. This man had raise him for 4 yrs now. Please help

The child's birth father would have to agree for an adoption... and your new husband will have to adopt him.

It sounds like there would be no objections from the biofather that would complicate the adoption process.

You don't file abandonment. After you are married your husband applies to adopt your child. The bio father will need to be contacted (or contact attempted) to get his permission. I would not attempt to do this without an attorney so you should start shopping around for one now.