If you want to start trying getting him to swallow cum this is totally the way to do it. Catch his cum in a shot glass as you ruin him the first time, put it aside and then keep playing with him and get him back to the edge.
THEN you had it to him and say ‘pour it in your mouth, but don’t swallow it’ and keep him on edge with his cum in his mouth for a minute or two.
Then you tell him to swallow it, when he’s super horny. Maybe you promise to let him cum if he does, maybe you just tell him to do it, or perhaps you just take him over and see if he swallows it in surprise!