I don't think it would be too heavy, but how thick is his coat? I have a pony with a fairly thick coat and his blanket is 240. It only gets about 10-20 F here at the coldest (maybe a little colder with the wind chill?). It's never made him remotely sweaty, but he always seems pretty warm under it. I'd be more concerned about the 640 d outer layer. I think anything under 1000 d rips fairly easily.