I'm still getting the 'perplexed, frustrated' face with my boy, as I've just gone the grazing muzzle route & he hasn't worked out how to eat through it! The pony is fine, as are all other horses that I've dealt with with grazing muzzles, but any tricks to help my boy learn would be appreciated!
Anyway, OP, yes, good thing to get him off the junk food, as the grass is probably more than enough for him anyway. A good supplement would be helpful, to give him what's missing in the grass. Lots of gentle exercise, building up gradually from only about half an hour or so, if he's really fat & struggling. If he's a 'lumpy bum' - got lumpy hard fat deposits & cresty neck, evidence suggests extra magnesium will help break it down. Take a look at safergrass.org feedXL.com & Dr Kellon's site, among others, for some good info on diet, dieting & nutrition. If the horse has been long-term obese, he may well(probably mildly at least) be laminitic, and hoofrehab.com is one site with heaps of info that should get you started there.
As for the ringbone, it depends where it is. If it's non articular, it may not effect him much, if at all, but if it's in the joint, this tends to be quite painful for them until it all ossifies & the joint fuses. Getting/keeping his feet functioning as well & naturally as possible(foreign to a lot of gaited & show horses) should help generally, tho it depends on what stage it's at, etc.