I don't remember exactly when Joey moved up to size 2, but if I were you, I would not buy anymore size ones. We got the big boxes of diapers (sizes 1 and 2) as a gift, and we didn't even use half the size ones. The 2s are gone, and my LO is like yours; big belly, skinny legs.
ETA: You get more size ones in whatever you buy, so I would use the coupon on size two diapers, even if you have to buy size ones "out of pocket".