The beetpulp puts a lot of water in them, that's why he filled out so quickly. What sweetfeed is he getting and how much? I'm asking because Arabians are, in general, easy keepers and get overweight really quick. It's so hard to get that excess weight off then plus they're one of the breeds prone to laminitis.
If he was mine, I'd switch him from sweetfeed to a ration balancer, together with the free choice hay and the beetpulp, and once he approaches ideal weight, would start cutting the beetpulp until he's at only a handfull with the ration balancer, which would be his steady diet.