What type/weight he may need will depend on what your winters are like and how he is housed. I'd go for a med weight and a heavy weight if you live in the "brrr" cold north. Or you could do a liner and a med weight. You need to check a few things like the denier(density I think).. The higher the warmer it will be.
Just poking around in Ebay
, here are a few examples. Kodiak 600D superior showman 1200 D saxon
Nice Denier McAlister 600D
I have one of these, love it.. This seller is very good also.
You just have to figure out what meets your needs. Do a bit of research via Ebay, for instance, to get some info on what is out there. Then figure out what you can afford that will do the job.