I think that he could use a little more weight on him, but he doesn't look bad. I would ad more hay to his diet.
As mentioned, if you are interested in switching up his feed I would suggest buying him a quality feed (Triple Crown, Buckeye, Tribute) and tossing out the sweet feed.
Alfalfa cubes are fine to feed. I have even used them as treats before. Try to, in the future, have a reason for changing a horses diet though
It wont hurt to give him this bag if you'd like to use them up or if you are set on keeping him on them. Alfalfa cubes can be fed dry or soaked but soaking them does two things 1) it prevents choking by making the cube easier to chew and 2) they soak up a ton of water so its a great way to get more water in him (which is NEVER a bad thing with horses) I feed them soaked. In a case like this I would probably feed 1 quart of cubes soaked in 1/2 gallon of water. I wonder why on one side of him I don't see any ribs at all yet on the other I can see a few ribs.
Most likely due to the way he is standing when you look at him, muscling differences (horses always have a "good" side) OR just his different coat patterns and the way white vrs black hides things. In this link, look at these horses. My horse gut is actually a little more full than these so I guess he's ok in that regard.
For future reference & for whatever its worth you CANNOT judge if a horse is under weight, at the proper weight or over weight by how "full" their gut appears. The weight that you are looking for is on his topline. Can you see ribs, can you feel ribs, how easily can you feel ribs, does his tail bone protrude or sit sunken in fat, hip bones etc. The size of a horse stomach will depend on just about every single factor OTHER than feed & weight. A horse in perfect weight and condition, well muscled and on a good work out will have a very nice tucked belly. That does not make that horse skinny. A horse of similar weight sitting in a field will have more of a gut, a broodmare can be thin with a huge belly, a malnourished horse can be thin with a big gut from improper feeding and or worms etc...
Take a look at this ponies tail head, hips, spine, ribs, withers and point of her shoulder, belly or no belly she is clearly thin.
Same mare 8 weeks later. Her belly actually went down and check the same weight points now...