Overweight horse...need suggestions

Alright, here's the deal. I have a 14.3 hand arabian that lives out in a pasture 24/7 with another horse up here in Michigan. Here's the problem. We've had a really wet, mild summer so the grass has stayed lush and green and consequently Rocky has been enjoying said grass and packing on the pounds. He gets no hay and only a handful of grain twice a day (i wouldnt give him any but its necessary to keep him from stealing his pasture mate's grain). I ride him 3 days a week, flatwork for about 30 minutes and then occasionally he will get lunged on a fourth day of the week. He gets the other 3 days of the week off.
I'm at my wits end here. He's still fat and I don't know what to do short of put a grazing muzzle on him. He can't go inside (he hates being stalled and will climb the walls and work himself up if kept in for long periods of time). Do you guys have any suggestions, maybe some great fat burning exercises for horses or something? Or do you think its time I put a grazing muzzle on him? I'd like to aviod that but at this point I don't want him getting any fatter and my riding seems to do nothing to that grass belly. Thanks in advance for any help =)

Make sure he's UTD on worming. Honestly if your feeding him just grass and a handful of grain it's for the best to get a muzzle. It's not cruel. To try and help him stall problem put him in the stall for 5 minutes one day, 10 another and so on.. as the days go on keep him in there for longer and longer.

he's up to date on worms and its def. Not a "wormy belly". I'm going to the tack store tomorrow to look at some muzzles. Any suggestions on the best one to get?

Oo and at this place he's pasture boarded only so I don't even have access to a stall.

feed hi fiber and low solube carbs NO SUGAR OR STARCH FEED an excelent source of fiberis mature grass hay NO ALFALFA try to stay away form meds and drugs and stuff and rapid weight loss could put ur horse at risk of hyperlipidemia get a muzzle for him it would be best

I didn't even know there were weight loss drugs for horses....wow. That just seems dangerous. As I said, I don't feed him hay and he gets next to no grain so those really aren't an issue. Hehe, and I guess this is vote number 2 for a muzzle

I would suggest when riding him do lots of jogging/trotting,I would also try a muzzle.I have a arabian that is also overweight but with riding and roundpen work he is slowing losing a little weight.

LOTS of exercise. That's what I've been doing with my 'couch'. He he.

Do you have any pics?

Going through the same thing with my mare except she was a pasture puff for the past 5 years with no work and got really obese and unhealthy she is now starting to get back into shape. She is on NO grass at this point just hay and her feed. I would definitely use a grazing muzzle don't want him to founder. Also what others have mentioned lots of trot work! Right now most of our work is trotting and it has helped a lot and I can see the difference.

The first thing I would do is find some way to restrain him from eating his pasture mates feed other than giving him some of his own.You could pen his pasture mate for the time that it takes him to eat his feed. (I do this twice a day for my old guy.) Then add in daily exercise.