He is on Seminole ration balancer. Pasture board 24 hrs. Has 4 other mates in pasture so a grazing muzzle will not work as he cannot defend himself and he cannot lick salt
First, Seminole - is that the 'Equalise'? Seems OK, as far as ration balancers go - it's low NSC, it's low dose rate, @ about 1lb per day for an average horse. BUT some things to consider - it has oils(fat) added, it has quite high Ca(esp in relation to the very low Mg). It has iron and potassium added, which can be problematic. And of course, as well as these specifics, any 'ration balancer' is a generic type supp, that may or may not have the appropriate amounts of specifics for your particular environment. Along with the Seminole, I would absolutely be giving extra Magnesium, to better balance the diet, but also for fat/IR horses, this can really help. This has been studied in regard to IR humans as well as horses & worth learning about.
If he's fat & on 24/7 (spring) pasture, you need to reduce his intake. I don't understand why you believe he can't have a grazing muzzle because he's in with other horses(& biting is OFFence, not DEfence), but that would be the best solution if it works, as it will reduce his intake but he won't be separated from his mates or locked up where he can't exercise himself. Otherwise you'll have to put him in a 'dry lot' or otherwise lock him up, for part time at least. As he will still need feed when locked up, you will need to get tested low NSC hay for him.
As for licking salt, if you're talking about a salt block, horses really don't get a lot from them, so it's best to provide loose salt or put it in his feed anyway.
Fat pads at his tail and all the other places.
Dunno what you mean by 'good neck', but if he has obvious 'fat pads', then I'd guess we're talking long term & bordering on obese, not just a little overweight.
So his topline is just starting to dip and a potbelly. I thought Cushings, so Vet is scheduled.
What makes you suspect cushings, aside from losing topline? Which of course will happen when he's not being as well exercised.
And yeah, exercise - I don't know how much exercise he gets in the paddock with his mates - depends on lay of the land, personality of horses, density of feed etc, but some horses will get a lot of exercise 'at home' but many, esp if it's a 'cushy' type paddock, won't get a lot. So more exercise will always be helpful. If you're considering locking him up, he will need you to provide a lot more exercise daily, to make up for this.