Very few cobs need haylege. Agree with the advice - muzzle in day, take it off at night (better to remove some of each day and the grass is lower sugar at night). Prob doesnt need anything like hay yet until the frost kills the grass. Then and only then regular but limited amounts of hay not haylege. There will still be old grass to eat. If you dont muzzle I wouldnt feed hay at all unless there is snow preventing access to the grass or it is really bare soil.
Good luck with fatty but do persevere - my pony got lami as I let her get too fat - dont make my mistake even if horse tells you they are desperate!
Then you need to muzzle and no rug. Are you able to ask the farmer if you can electric fence off a section for him? If not you may have to consider moving yards.
I agree with what has already been said. You need to stop the hayledge, whether it means muzzling or sectioning off a bit of the field or move yards if you cant.
Why cant you exercise the horse? Theres plenty you can do to get your horse moving, even if its just walking in hand etc. Its the same with horses as people, eat less move move. Good luck x
Lunge him regularly. Restrict grazing. Give hay instead of haylage.
If there is 30 horses on 9 fields - then basically its clearly too much for your horse as he's too fat (which he shouldnt have been allowed to get in the first place). You need to section off a corner of a field or something for him and then you could give hay instead. If the YO wont allow it, move yards.