The cougar thing had be expecting at least a 20 year gap too. Although I guess to some people 10 years is a big difference, especially that way.
Anyways, is he going to stay your client for a long time? And his mom? My main concern would be that he'd be awkward around you if he says no, and you around his mom is he says yes.
I think if you tell him, do it when he has the option to leave right away, so he doesn't feel trapped, but can also stay to talk more about it if he wants to. What does your husband think? As he always had partners who knew he was poly from the start too?