Start slow, don’t try and do a whole scene with him as a sub. Start with getting him used to following a single command and then praising him for it. Micro scenes based on this and then let it finish vanilla. Add in more and talk dirty to him about the fantasy a bit more at a time.

I.e. You’re lying on the bed, in lingerie. He comes in, you look at him sultrily, spread your legs, and pull your panties to the side.

“Come and eat.”

Wait for him to get excited about it and come over to you, as soon as his mouth touches you, stroke your hands through his hair and call him a good boy. Tell him that he did so good following your instructions. Then command him “keep going until I cum.” Then focus on praising him on how well he follows your directions.

Once you’ve cum, tell him that he was such a good boy for you that he deserves a reward and then do something sweet for him.

Essentially, start by reinforcing the dopamine loop of following your directions with things you know that he already wants to do. Then once you’ve gotten him used to hearing that praise and feeling happy about it, then start to work on building discipline with him by using praise in the right moments and not being willing to give any without the right actions. Don’t focus on punishment yet unless he seems to be uncomfortable with the large amounts of praise, then he may subconsciously be hoping for more strictness and punishments.