Don't buy him a dog, or get one from the shelter for him---let him make this journey on his own, when he's ready. If he doesn't have the money, offer to give him $X toward buying a puppy from a breeder if you want.
And, to my knowledge, no rescue group with integrity will allow you to adopt a dog on someone else's behalf (unless maybe it's your child), because they want to check out the person who is going to be the primary caregiver (your FIL) and not you--he has to pass their criteria, not you, so I don't believe you can do it for hm. For example, some of the breed specific adoption groups require home inspections and veterinary references, so they would have to check his house and talk to his vet.
If he is really set on jumping into another relationship with a dog, print off some contact info for various sources (breeders, shelters, Weinaraner rescues), and let him take his own path. Be supportive, but don't do it for him. :)