never cut off their food and exercise like this. if they are use to being out 24/7 you need to slowly ween them off the grass. they still need hay in small portions throughout the day. shavings for bedding
autard I don't think muzzles are that nice. And I ride every evening I have to much grass tho.
stabling them may be good but it will also help them out to work them! put them in turnout with a muzzle on if they eat too much grass or even just lunge them for 15-20 minutes a day!
shavings or he could eat the straw