I have dealt with many skinny horses and horses with tummy issues. My farm rescued a very neglected and emaciated 30+ year old gelding in the fall and he is now doing great despite being unrideable from permanent lameness. It sound like your pony is giving you a run for your money! I have had great success with boosters of regular deworming, gut health products, and LOADS of food!
To help his belly and get him to eat better, 1: Give him a big dose (a couple hundred lbs. more than he actually weighs) of IVERMECTIN dewormer (make sure it is IVERMECTIN because not all types of dewormer are safe to over dose) ASAP and then deworm him accurately with other types every 4-6 weeks with permission from your vet P.S. try to re-dose with IVERMECTIN once, 2: call smartpak and order a few rounds of gastro guard and ulcer guard. Administer them as directed by your vet which will most likely be regular doses for a course of several weeks. The gastro/ulcer guard will rid him of ulcers etc. but I would start him on Smartdigest ultra instead of Smartgut because it will support his entire digestion system AND help prevent the cured ulcers from returning instead of focusing on mainly ulcer prevention. Plus the colic support program (ColiCare) that comes with the supplement.
For weight gain, I would have him on 3 lbs. of a high fat fortified grain (performance and senior are often great for this) 2x a day, plus two 3 qt. scoops of alfalfa cubes and 1-2 lbs. of hay stretcher (turned to mush) either at each meal or as lunch. Give him literally as much hay as he can eat and encourage him to eat more! For the water electrolytes are great but his belly feeling better and his condition improving will also help greatly. If you wanted to put him on a weight gain supplement, there are many good ones (cool calories, smartgain, farnam products etc.) out there but with correct feeding are not always necessary. If it is cold, significant blanketing can help because it keeps them warm so they do not burn through as many calories and use their minimal fat. If it is warm, hours of grass pasture will help a lot! As always, run everything past your vet and use your knowledge of your horse. These are my suggestions and tips I have had success with.
-Hope this helps, good luck!!!
The triple threat of riding = EVENTING! 😉