he is about 14 hh... the vet said on the scale of 1-9 he is about a 3... and he measured him and everything he said he is about 710 and he should be about 800 - 810... It is 12% sweet feed, I give him one scoop twice a day and I measured it a full scoop is about 4 lbs of feed. He was getting a little scoop twice a day before I got him so I slowly worked him up to a bigger scoop because I feed both of my guys with the big scoops. I'm thinking of switching him to oats but im not sure if I should? What is the best thing to have him on during the winter to keep his weight up? And then during the summer since he is out on full pasture all day I know some people don't even feed grain during the summer just hay? If you want I can post a picture of him and you can see what he looks like... his ribs show a little bit my worry is his back bone looks like it needs some substance of something around it.. it sticks out a bit too but he has a belly.. I don't get it...?
Yeah, I'd ditch the sweetfeed for a start. Straight oats(pref not whole) are apparently about the safest grain for horses, but they're still starchy grain, so I would personally still be looking at healthier options & not feeding grain unless they really need it & there was no better alternative(of which there usually is). Again, especially if you are going to feed starchy feed, it is best to feed little & often, rather than only a couple of large meals, to minimise problems & maximise the goodness the horse can get out of it.
I'm no nutritionist at all tho, having only studied it roughly, so do look into it yourself. You can find a lot of info online regarding healthy feeding & the probs with starchy/sugary feeds, but I also recommend people find a good equine nutritionist or service, so they can consult them & get the facts. I would particularly look into insulin resistance, hind gut acidosis & associated weightloss, to rule that out & avoid it. I personally subscribe to FeedXL.com which is a great service, which also has a great program for analysing & working out custom diets for individual horses.
Regarding winter feeding, digestion of forage, not grain is what generates body heat & free choice hay is generally fine. Including some alfalfa/lucerne is generally good too. If the horse does need extra fat, mixing a bit of oil(pref cold pressed flax or such), oil seeds or such with their supplements, or including a low(or no) starch, high fat feed may be all that's needed.
Regarding what he looks like, yeah, can't say much without pics at least, but it doesn't *necessarily* mean a horse is underweight if you can see a bit of rib. Likewise his backbone sticking up *could* be lack of muscle, rather than too thin. Having a big belly immediately makes me think worms are a possibility. I assumed you would have wormed him & had his teeth seen to, but if he did have a big worm burden, it may pay to do him a few times. Your vet should advise you there. The big belly could also be to do with digesting starch.
Oh and regarding your other horse 'inhaling' hay
, you could put it in a doubled or small holed hay net to slow him down, or look into other 'slow feeder' ideas.