Hello and welcome to YT!
I'm afraid I don't know too much but I have read elsewhere that thin hair could mean a thyroid problem so check this out with your vet. In terms of his size and again from what I've read, his weight now at eight month's is pretty much is grown weight give or take half a pound or a pound. So you're probably looking at 4 or 4 1/2 at the very most. Hope that helps and please try post pics of your baby if and when you can. It'd be lovely to see him
Annie x