I definitely agree on worming him ASAP. Pot belly plus ribs means worms.
When we bought our emaciated 5 yr. Old TB gelding from a used horse dealer, the FIRST step to weight gain was to put him on Continuex for a few months. A daily dewormer. This clears the system of ALL stages of worm cycles, whereas a paste dewormer often only kills the adult worms and not the larvae. Once you have cleared the system with Continuex, you can then start with the paste dewormers. Using a rotational worming system of course.
There are many types of paste dewormers out there and they can be used in a 2 or 3 way rotation...check with your vet about tapeworms. Certain areas of the country have a high incidence of them and ivermectin, pyrantel, etc dewormers DO NOT kill tapeworms. To kill them you will need a Praziquantel dewormer.... I personally ONLY use Zimecterin Gold. Equi max has some controversy surrounding it's safety level. IMO, any horse with signs of worminess, needs a daily dewormer for a few months....one dose of a paste dewormer is not always sufficient enough to kill the worms in their larval stages, killing only the adults, whereas the larvae then hatch and your horse is in a vicious cycle and never completely free of worms...despite the paste deworming having just been done.
I also HIGHLY reccomend Cool Calories 100 for weight gain and coat conditioning. We turned our boy from this:
To this, in only 7 months:
First step was a daily dewormer. He gets 3.5 lbs of Strategy once a day, a probiotic/prebiotic supplement, and two scoops of Cool Calories 100....he gets only limited GRASS turnout (about an hour a day) due to circumstances forcing him to be on dry lot, and one and a half flakes of hay in the AM and one and a half flakes in the pm. Even on dry lot, he's blossomed.... his coat is super shiny and so soft that his trainer stopped right in the middle of a lesson last night to touch him and comment on his soft coat. And since the Cool Calories, he's really packed on the pounds fast....
We'll keep him on it through the winter, but come spring, we may need to adjust the Cool Calories down....or he's going to end up FAT.
Your horse may not be touching his hay because he's on grass, and given the choice, EVERY horse will choose grass.....but the ribby pot belly is oftentimes indicative of worms, and you will want to get that cleared up...because once you do, he will really feel and look better.
Also, if you need weight gain or coat improvement....DEFINATELY try the Cool Calories 100.... it has worked miracles for our boy. As proof of it's miraculous power, it turned our TB into a little pudgy boy DESPITE being on dry lot....oh, and the Cool Calories 100 has NOT made him one bit hot. He's still the same sweet, calm and sane boy he's always been.
Since starting the Cool Calories, the change in his appearance has been truly remarkable.