Is this for his cot?
Oliver had a 4.5tog for his cot, but now has a 4tog for his bed. He wears his full pj's and socks, has 2 primarks blankies too but still kicks the covers off..I've just been up and he was a tad warm, but is room is at 22c.
I didn't think you could get higher tog duvets for cots tbh.
If I were you i'd just offer the extra blankets for now until he goes in his big bed maybe