I would think a ration balancer would be better since he's not getting any other hard feed. I have a hard time believing that multivitamins can pack in everything a horse is missing from grass/hay alone in a tiny little 2 oz. scoop, yet they typically cost more to feed per day than a ration balancer.
Here's the guaranteed analysis for Dynamite, a super expensive multivitamin that I foolishly bought for a horse I brought home on trial in December (and promptly sent back) https://www.securedcontent.net/dynam..._SuppFacts.jpg
And here's the guaranteed analysis for Triple Crown 30% Supplement (a RB meant to be fed with grass hay): Equine Supplement-Triple Crown Pelleted 30% Equine Supplement
Percentages can be a little tricky, as you have to remember that the Dynamite is fed only 1 oz per day, while the TC30 is fed 16 oz per day for the same horse. So for example, Dynamite has min 3.5% calcium, which means in a 1 oz serving, the horse is getting .035 oz of calcium. TC30 is min 4% calcium, so the horse gets 0.64 oz of calcium in his daily ration, 18 times as much! Either TC30 is WAY overdoing it, or the multivitamin simply can't provide enough in so small a package. I think it's the latter.
Then there's the Biotin in Dynamite which is measured in MICROgrams per pound. At such a small concentration, it seems like it's there more to fill out a checkbox than to actually be useful.
My 2 cents