I wanted to ask you all a question about my horse, Storm. He is a just-turned 3 yr. Old Tennessee Walker and a real sweetheart. We purchased him along with our other horse, Sarge, from a farm nearby. However, unlike our other horse, Storm had not been with the farm very long at all (only about 2 or 3 days) as he was rescued from owners who apparently did not feed him properly as he is a bit emaciated.

He has a large pasture to do serious grass-grazing during the day and of course hay and treats (apples, carrots, flavored horse treats). In addition, we've been keeping up with the same feed he was given from the farm (Reliance) but I've been told that that brand of feed is very hard to come by (and I have yet to find it in my area) and I'm getting near the end of the bag. I dont' want to make the horses colicky and wondered how to go about switching them to a new feed and what feed would you reccommend trying for a horse needing to gain weight?
I realize a Tennessee Walker's frame is not meant to be as hulky as a QH, but as you can see our boy's ribs show very easily and you can feel them like mad when you are grooming him. He's just a dream horse---couldn't ask for a sweetier, gentler boy, and we want to get him at a proper weight for his size.
My questions are---what weight should he be at? How much does it look like he should gain? How long will it take him to gain the weight?