if you double up on the batting it is going to be thicker/puffier on one side- which may make a lop-sided quilt- but if you don't mind that give it a try- it would be easy enough to make a backing that was cotton/flannel or even fleece (very warm & cozy) you could also add warm (features) to the top on his side- like flannel, wools or corduroy appliques that would add additional warmth/weight. try a wool batting too- wool is wonderful batting - is lightweight, breaths, tends to (magically) adjust to your warmth.