I'm so confused about what size I should be buying/should have bought for the winter, and maybe a more experienced mom can help me out here! Especially while I'm still in the return window for a lot of things.

So J is 10 weeks old today and has grown 5 inches...I don't know how but his ped says he's at the 86th percentile for height? That baffles me but okay. He's pretty skinny though so waistbands are a struggle. He's wearing 6/3-6 onesies and rompers and not quite there with 3-6 regular clothes. Does 6-12 sound right for winter?