I am working with a 22 year old Arabian mare that needs weight too.  I have been successful finally by adding corn oil to her diet.  I started out with 1/4 cup, and every week raise the qty the same until reaching 1 full cup. 

I also am feeding timothy pellets dry, as much as they want, and also giving 2 large scoops approx 4 lbs of senior, 8 oz of rice bran every day.

I just added Revolution Rx to the mix and will cut down on the senior as I work my way up to 3 lbs per day of the Rx. 

I have noticed that since adding the corn oil, that started to make a difference.  The calories in the oil are huge!

The Rx is some new product that helps the horse digest and absorb the food while in the digestive system, and it reduces manure because the food is getting digested more.  The manure is also drier than before and a darker color.

The hair became more shiny...not sure if that's the oil or the Rx.

Either way...I am seeing results!  I notice the rear flank filling in and the ribs not showing anymore.  Luckily the horse has a big appetite and can eat everything I give her. 

Lastly, check with your vet!  I always make sure to consult with the vet first before I do anything. 

If you have already checked teeth, and done a $25 fecal test for parasites...then it's all about the food.  My vet clinic does the fecal test in like two hours.  My horses are clean:)

Keep me posted!  I am always looking for new ways to fatten up a skinny horse.

Best regards,

Eric

If he's really hyper, then I would cut out all of the Alfalfa.  Switch him to bermuda or Timothy hay.  Both are high in nutrients, but lower in digestable energy.  Feed him as much as he'll eat, at least 2.5% of your goal weight for him in hay a day.

For weight and to complete his nutrients, feed him a ration balancer, like Triple Crown 30%, with a probiotic for better digestion (like Source Focus WT), and a fat source like stabilized rice bran or flax (meal or oil).  

This combination worked VERY well for my flighty and skinny Anglo Arabian (TBxArab).  I added back in some Alfalfa pellets, but in small quantities (3-5 lbs a day), when he's in full training.

Make sure his teeth are in good shape, and have wolf teeth pulled if he still has them.  I would also deworm him with ComboCare now, Ivermectin in 8 weeks, and TapeCare+ 4-6 weeks after that, to make sure he's parasite free.  After that, do fecal egg checks and keep him on an appropriate deworming schedule.

Ulcers can be another cause of weight loss or low weight.  You might ask your vet about that.