...tell me about their sizing from winter to winter?

DS is 18 months old and still wears a combo of 12mo and 18mo clothes. We just bought him a coat and thinking it was 18mo, and got it even though it looks big. Turns out the hanger was wrong and it's 2T... Trying to figure out the odds he can wear it next winter if not, DS #2 will eventually get to use it, but wondering if we can get 3 seasons use between the 2 kids