I have a (treated) PPID and IR horse, and I still let her out to graze for 1-3 hours (depending on the time of year) with a grazing muzzle on. It keeps her sane and much happier. She needs to run and act like a horse sometimes. The only time I cut it off is if her body fat shows me that she could use a few days off the grass, typically during the seasonal rise. If you slowly introduce grass to your horse, since it’s been getting none, it’d be good for him!