He's in fantastic shape! I wouldn't want to see him LOSE any weight, but I think 100lbs may be a stretch. If you can keep him in this place, he looks fantastic! I know the difference of 100lbs on my little Arab (with much the same body type) is the difference between skinny and obese - 100lbs may not matter on a 1500 pound Draft, but on a smaller horse that's quite a bit and it would make him overweight.
MAYBE 50 pounds if you want to be really picky. He looks ideal to me!
Too many people think a fat horse is a healthy horse these days but really it's not true. I prefer a little bit of plumpy-ness on my horses but owning an overweight horse (believed to be insulin resistent) w/ cresty neck and all, I don't like to push it.