Does he have a crest developing on his neck? This is one of the most obvious signs, btu doesn't appear in every horse. If you can feel heat in his feet then he is CLOSE to actual founder and get him off that grass immediately.
There are horses who founder, and there are horses who don't. Shetlands and ponies are more prone to it that other breeds, but other breeds do founder nonetheless. My 14.1h arab gets fat as billy-o but he has never even looked like foundering. It really depends on the individual horse.
The best thing you can do is give him regular excersize. Is it possible to tape off a portion of his paddock and eat it down? This would give you a barer yard to maybe put him in at night to reduce his intake of grass. If it is bad you could let him out for only an hour or two. My first pony, a welsh mountain, had to be locked up at night in a dirt yard to keep her at a healthy weight.
To me, muzzling is at the extreme end of the scale. I don't really like the idea of muzzling my horse... lol. They wouldn't be able to itch themselves or groom with buddies...
Most founder prone horses can be managed with periods on grass and periods locked up. A shetland at our place at the moment is kept on dirt and only allowed on grass an hour or so a day. She is a very healthy weight and doing great.
There are also products that claim to help prevent founder. I have used them, but have no idea wether they are effective or not. The one I have experience with is founder-guard. Link
My reccomendation would be to manage his weight with excersize and periods of grazing/periods of non grazing time.