Yes... Both parents MUST agree to an adoption... In doing so dad would be giving up all legal & parental responsibility & you would be taking that on... It's a big deal, for the parents, the adopting parent & the child...
Your business, but if dad is not in the child's life of his own choosing, is not causing problems, but is paying what is required of him in child support, then one would have to ask why on earth would you feel the need to adopt. Mum will lose her child support.. What's to be gained?