Too hot?

I feed my son to sleep on a pillow and recently his head has been getting sweaty. I’ve checked his back and chest and they’re cool to touch (I.e. not unusually warm as I’d expect from the sweat). Our room is 18-20° and he usually sleeps in a long sleeved vest and a 2.5 tog. Is it worth switching to a 1 tog, do we reckon? I worry about him being too cold then 🤪🙄 especially at 4/5am where I can feel the chill in the room.