We wree recently talkign at thebabywearer about how Mexican women wear their little ones up to age 9 or so in 1 shoulder carries in rebozos while at market. Your son is so lucky to have a mama like you who cares so much for him!!!
I second and third the suggestion of a wrap carrier. I wore my son on my back at a week old, with almost no muscle control. The piggyback position of teh back WCC would be comfortablr for a larger child ... I still carry my 40+ pound 4yo this way. You can spread the fabric to cover his arms and back up to his neck to support him.
Normally I would heartily recommend gauze to you; I love gauze wraps beyond all else, and think they're great. But the fabric of most gauzes is thin; I would recommend either a heavy gauze *they;re out there; I have a few lovely thick ones) or a heavier fabric. I wouldn't think a Didymos would be what youwant; they're 22" wide and that wouldn't be enough to tuck in an older child well in a back wrap cross carry. A guatemalan hand-woven would be lovely, though expensive. They usually come in 30" wodths, and at our local fabri cstore they're $15-#25 a yard. You could eaily hem of fringe the edges yourself. I think EllaRoos are about 30" and cost about the same, and have no sewing either, just fringed and knotted ends. And since there are no seaams in a wrap carrying heasvier childern is less of a concern then in carriers w/seams.
I hope this helps ...