Hello and Welcome. Please take your time to read and consider all replies as they are carefully structured with your best interests in mind.
Your decision should be based on the most relevant and important factors only and in your case, that is your son and the dog as they are an established part of your family. The boyfriend is not. Your son and his dog rely on you for the security and love you provide as the mother. The boyfriend does not. Therefore it is not unreasonable to choose your son's dog over the boyfriend. Your son and the dog probably have far more to give you in return. Give yourself credit for already having stood your ground in retrieving the poor dog from the trash. Hopefully your son and his dog will not have to be subjected to repeated episodes of cruelty and distress. Your decision might alleviate that possibility.