I agree with the above, having your vet on board will help rule out potential causes, and will also help in the event that something happens, he knows what is going on with your horse, so you don't have to go over everything when he comes out. If he's not on a complete senior feed, I'd put him on one.
Our gelding at 35 didn't have any problems eating hay, but its a common problem in older horses, so the pellets should be soaked. Beet pulp is a good additive to help put on weight as well, and you could also potentially use some sort of weight gain supplement, though I'd be cautious about that until you get a vet out to rule out any causes.
I would start blanketing anytime the temp is under 60, and adjust the blanket weight as it gets colder, to help add an extra layer of protection. I never blanketed my younger mare unless I had body shaved her, but we blanketed my dad's horse every winter, and it doesn't get too cold here in southern CA, but I believe it made a huge difference in him keeping on weight, and being less stiff, and overall happier.
At the very least, I'd talk with your vet over the phone, and just ask what he/she thinks, if you absolutely can't afford to have him/her come out. Good luck, hope that he gets better.