Do you have any photos? What would you say he'd be on the henneke scale? Henneke Body Scoring
If he's not a working horse, he'll do just fine on pasture. If his weight is a big concern, you could take him for 15-20 minute walks a few times a week (you'd be surprised at how much weight a horse can lose on just walking alone). Although you say he's not necessarily obese...cutting the grain could do the trick to plateau his weight.
How many hours is he on pasture a day approximately? Also, what grain is he on?