Hi, firstly, without knowing what your hay & the grass is, can't say whether they're any different except for water content - commercially grown hay is usually from reasonably rich grass.
If the feed - grass, hay, grain... Is too much for the horse, in amount, sugars & also particularly if the horse / diet is low in Mg, they tend to develop fat pads when they get more than a little overweight, usually including a cresty neck. This is a sign of IR/EMS & not something that happens suddenly. Certainly not an early sign to wait for before being concerned. *Altho high phosphate feed(eg feeding on newly fertilised grass) & sudden or severe Mg deficiency can IME cause fatty deposits to become suddenly hard.
Re his hay belly, 'ridiculous' energy & diet, I'd first be looking at his condition & whether he needs that much feed for a start. You're right that sweet feeds aren't great, but safe choice isn't a really low starch/energy feed & he may not need any extra to the grass at all anyway. His excessive energy, 'sensitive tummy' & hay belly may also be due to the starch levels in his diet and magnesium imbalance is another possibility that comes to mind, which can cause nervy behaviour. Extra Mg in the diet can be very beneficial for a number of reasons & well worth looking into.
I also wonder, did you re-evaluate the nutritional supps he's on considering his changed diet, to ensure they're all still appropriate for him currently? If not & not sure how best to do that, FeedXL Horse Nutrition: The D.I.Y. Equine diet planner
is one excellent program/source of info for all that. Posted via Mobile Device