OP: Try to find out his deworming history. If it is lacking, then go slowly. Its tempting to throw a lot of dewormers at them if you know they're wormy, but this can cause problems. The best idea is to take a fecal sample to the vet so they can decide which worms he has and treat appropriately. Since you'll likely be doing a pre-purchase and having them out for a Coggins draw, you can do it then!
Originally Posted by palogal
I would definitely take him off the round bale. The vets in my area jump all over people that have horses on round bales.
What?? Why? If he's underweight a round bale is probably a good thing for him. My TB gets pretty ribby in the winter and you can throw all the omelene, sweet feed, senior, etc. you want at him but it really just makes him hot, which makes him pace more and sweat more and lose more weight. A round bale is the only way I keep weight on him. A horse can't colic himself on too much hay. The best way to get weight on a horse is more hay and maybe some rice bran which you also cannot colic a horse on. It's the sugars in a lot of the "performance feeds" that get you into trouble.
PS I'm a vet, and a lot of the other vets I know also love round bales for hard keepers.