Aside from the usual "have the vet check his teeth" etc, I would say it's quite possible he just needs more hay, if he's able to chew it, rather than more sugary sweet feeds. Calories from fiber do more for a horse than sugar and starch.
Older horses start to loose muscle tone as they age, mostly around the topline, and at some point, you simply can't get away from that ribby look in some old ones. I have one that is 26 and perpetually ribby, and swaybacked, with a prominant spine, but the vet says' he is healthy, just old. If you've had your horse's teeth worked on, not wormy or suffering from any other obvious health issues, it could be he's jus turned that corner in his life.
Oh, and personally I'm not a fan of over-worming horses, and it can be hard on some them. The deworming could actually contribute to digestive problems (killing off healthy gut flora), so deworming only when a need presents itself and feeding a pro-biotic can go a long way to help him out.
Also, some people feed bute on a frequent basis to older horses for arthritis or whatever, and that could cause ulcers, which in turn could affect his weight, so use them judiciously if you need to administer them at all.