I have tried beet pulp with good results in gainig weight. The only problem is that it tends to make them hot and stiff.

Looks like it will be rice bran and the alfalfa.

Pre/ probiotic fast track or FORCO. 1/4 cup coconut oil 2 tablespoons white salt, clovite, Purina calf manna( designed for horses)

My feed program for years has been good alfalfa, ration balancer and rice bran. I feed it to everything, young/old, hard keepers/easy keepers, pasture ornaments/perf horses/horses in training, never once have I had an issue. I don't buy the pellets I get the actual powder stuff as the process for extruding pellets requires heating which changes nutritional value.  I feed  2 lbs a day making a mash. I've never had an issue with it turning bad as I keep it in a sealed container  and use it quickly even in the summer months. My horses love it and if they need meds or something it's easy to just mix it in with zero issues. I've tried some of the other more expensive mentions on here and as someone pointed out, mostly fillers. RG May very well be a great product but it did not work for me, expensive and the first ingredient is rice bran. 

Increasing high-quality forage always does the trick for me unless there's an underlying medical condition. Are you able to offer free choice high quality forage?

I just started mine on this to give it a try. Not a young horse, but just wanted some extra calories coming out of a hard winter for my 7 year old. She doesn't tend to carry much extra weight anyway - she self regulates.

Test your hay and see what it's providing, you'll have a base line that way. Get ahold of some good- GREAT alfalfa (i prefer long leaf for gut health/saliva production) which will provide a lot of calories, then i feed a locked ration fortifed feed. If I didn't have access to Bluebonnet, I would feed Triple Crown.   I have so many before and afters of my horses and customer horses after making the switch. Depending on how much weight you are talking you may just consider the HindGut Program (if your hay is able to do the job from a calorie basis).

The answer is a good all around feed....feed 2x a day, free choice hay and clean water. After feeding twice a day for a month without results, increase your feed rations....this of course if you've wormed and checked fecal on horse first. If this isn't working, check their teeth with equine dentist. If that doesn't work, go to the vet and check for ulcers. If horse is healthy and you're feeding religiously same time every day good quality feed, he will gain weight.