Well communication is key -

1. Ask him if he likes doing it - ask him if he likes doing (or would like to try)anything else.

2. Let him know that you like it (yeah he does know but make it explicit).

3. Talk to him, about experimenting with things... (this is expecially helpful if he has already answered #1 with other things that he would like to do)

5. Mention something outside of what you do with him... i.e. wax, bondage etc.

4. Buy him Easton and Hardy’s topping book. ( I think they have a new one out)