May I ask if he has explained why 15 is the lower limit? I personally am not into age play, though I can understand the attraction of doing something like a naughty college/university student scene or something of that nature.

However, due to the experience of a close friend being raped, I have some very strong feelings on consent. For me, these also tie into matters of age, and I'm literally clenching my hands into fists as I type.

It may be a similar matter for him. If 15 is as young as he is comfortable with, it is unlikely to be an arbitrary barrier. He may feel that is as young as someone can be and still give informed (if not legally recognised) consent. Going below that may, for him, feel less like age play as an enjoyable activity and more like he is, on some level, pretending to be a paedophile.

I would try talking to him about it. You may not like being told "no", but at least you will know why, and that should at least make it easier to bear.