That's it. Start nice and slow, getting your son comfortable as you stroke his ass before fingering his hole. Then slide your cock into him and proceed to fuck him. But why did you pull out when you came? You should have kept fucking his ass until you came in him and pumped all your seed deep into his willing submissive body where it belonged and will help to further feminize him. That's the idea, isn't it? To make him into a submissive bottom for you to fuck.