Since you are going to be helping him explore his fantasies then you have the person who can help you figure things out right there. Him.

If he can't/won't/hesitates about expressing his fantasies then look at the pictures you said he has on his hard drive, preferably look at them together, and find out which pictures he likes more than others and why. From there ask him to tell you which stories he has read on the site that he liked and with those two bits of knowledge in hand you should be able to build up quite a satisfying scene.

A note about hypnofetishists, and mc types in general, part of the kick they derive is from a loss of control. Different from a more traditional submissive who participates in power exchange where control is relinquished. With this in mind I would focus on the "loss of control" aspect reinforcing for him that he is "under your power and control" may be the real kick he is looking for.

Drake.