My son usually sleeps in a one piece pajama. When it is cold, I put him in a thick one, and when it warms up, I switch him to cotton. He always sleeps with an undershirt. Carters and Macys sell one piece pajamas for 24 month olds and Ive seen them for even older kids. My son does not sleep with a blanket, and a sleep sack would never work for him. It is amazing how much they move around when they sleep. I will put him to bed on one side of the crib, two hours later hell be sideways in the middle, and later hell be backwards facing the other way! These little guys are notorious for this, and he never wakes up when he is doing it! In the morning, he hangs out in my bed sometimes and Ill place a blanket over him and he immediately kicks it off, so I havent bothered to put one in his crib (with all that movement, he might get wrapped up in it anyway..). You may want to consider removing the sleep sack for a couple of days, just to see how your son reacts. Put him in a warm onesie, or warm two piece pajamas with socks (I put the socks over his pants sometimes so he doesnt kick them off at night) and just lay him in the crib without a blanket or sleep sack. It might work! Good luck.