If everyone else is buying a round, call him out for the next one while the rest can hear you: "I think next one's on you". Works for me to push the less inclined. Also good to stress any rounds that are given: " I'm buying the first round" etc. If he still doesn't follow, don't buy for him. If he asks, tell him you didn't get a round from him yet, so you thought he wasn't drinking anymore or smth.