Far cheaper option is to buy a bag of milk powder, the type you can feed to calves, goats, puppies pretty much any animal really, will last longer, achieve weight gain and not cost you so much.
Working well for my TB and we used to feed the point-to-pointers on it.
The one I am using is made by SCA, long established firm from somewhere in North Yorkshire. Most agricultural merchants will stock it. It is a pure milk powder, doesnt have any growth promotors etc in it.
Smells very like the jelly sweets called milk bottles only more concentrated. My boy loves it mixed with warm water and just tipped into his feed bowl, failing that he likes to drink it out of the mixing jug!
A tub of EQUIVITE Body Builder should last about a month. It's great stuff.
I wouldn't use something formulated for cattle or sheep on my horse, their digestive systems are completely different!! Why take the risk of upsetting it?
If your horse is already slightly lacking condition I really wouldn't use something that is specifically for a horse. Why don't you try a tub of EQUIVITE Body Builder and see if it works for yourself first.
The SCA milk is safe for horses, quite agree you should not go for calf/sheep milk. Years ago before all these fancy feed suppliments milk powder was regularly fed to horses.