Sooooo cute!! We suggest size 6-12 months and it fits your 10 month old perfectly and he has chubby little thighs and arms. It is more of a t-shirt fabric than it is sweatshirt. You can read the reviews before purchasing and knew to expect that--which is fine because that's what you were looking for. Your son can wear it Spring, Summer, Fall and Winter. He wears a onesie underneath it. The length of the shirt is great too. Goes to about the middle of his tush. The camo colors are bright against the black. I did buy this in 12-18 months to because I loved it so much and it's pretty big on him so I say this outfit fits true to size/expected. I HIGHLY recommend you get this for your boy(s).