I think you will be pleasantly surprised in how well diligent hand walking can be for both his weight, muscle mass and hoof health.
I would start out hand walking 20-30 minutes a day. Not a leisurely stroll, more like a power walk. Just do this for a few weeks. He should be conditioned enough to take a rider by then. Then once you start riding continue with the walking for about a week and then add in some trotting.
As far as feed goes- no grain. Stick with hay. If the hay is lacking any nutrients, you might add in a ration balancer. A slow feed hay net might not be a bad idea, either.
For his white line/ hoof issues- Spray on some apple cider vinegar every time you pick his hooves. It should keep any funk at bay.
Walking, got it. Any work at his extra gait? He step-paces. And apple cider vinegar? Makes sense, thanks a lot :) That's been bugging me.