Gaining

Just feed him and gauge his reaction to the results over time. You'll find out how receptive he is to gaining without starting an awkward conversation.

Be sure to tell him how attracted to him you are. And don't be ambiguous about it.

A friend of mine once said that the thought most guys, on some level (some deeper than others) are gainers. So just tell him you think his chub is sexy and you'd like to see if he can gain a little more weight.