Lots of good and interesting advice here. I may have missed it, but was there a mention of Will's other behavior last year, and throughout the year, regarding the OP? If someone doesn't literally say "thank you" upon a receiving a gift, but it is pleasant and friendly, and makes an effort to keep in touch with me, that would seem more positive to me. But if someone basically grabbed the gift and ran into the corner, and I hadn't heard anything at all from him since, that wouldn't be cool in my book.
But, I do like to give people a second chance at least, because anyone can have an off day. I like the idea of giving Will a gift equal to Riley's and then engaging him in conversation about it. Not only would that forge a bond in general, it would allow him to show his appreciation of the gift even if he didn't actually say "thanks."
And in this particular situation, with all the backstory, I admit that I would feel sorry for Will, and would probably try to give him more chances even if he didn't respond "the right way." That doesn't mean I would spend a ton of money on him each year, though. But maybe I would keep giving him $25 cash, and I would give Riley $25 cash at the same time; but maybe a couple weeks earlier I would've taken Riley to lunch, or otherwise "gifted" him away from the holidays, because I feel closer to him and he makes more of an effort to be closer to me.