Roro is correct. Sometimes when I see questions about a horses weight, often people suggest giving more or different grain. But generally, it is not grain that puts weight on, its the hay
As for the abdominal strengthening exercises, I do those to my horse too. They were suggested to me by my equine chiropractor (Grady being off the track, he needs to get realigned sometimes). She did tell me however, only to do a couple on each side at a time. Lifting the abdominal muscles like that is hard for work a horse!
And finally, someone once told me that if a horse is a little ribby but with a heavy belly (similarly to your horse), that means they could have worms. Is that true? Could be a question for another thread, but I thought I'd ask!!
"There is something about the outside of a horse that is good for the inside of man." ~ Sir Winston Churchill