Need to put weight on my ew Welsh Stallion.?

9 year old Welsh stud just purchased from a Amish man...Hasn't had grain in 3+ years. NEEDS to be bulked up. He's on 65+ acres of grass which sometimes makes it impossible to catch him, so feeding 1 time a day is what will be taking place. Any idea what to put him on so he doesn't colic, but gets weight fast?
Answers:
I do not recommend 1/day feedings. I know ppl do those, but horses have a higher chance of colicking and ulcers. You really need to turn him out to pasture if thats your set feeding schedule then.
Horses were not meant to be fed one/day, they are grazers and their whole system is set to that.
Anyway, 2x/day feedings, worming, equine senior, quality hay and beet pulp to bulk him up.

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I am apretty horsey person, so I say no grain. Oats are pretty easy on their tummies. Ponies do poorly on grain, they colic and tie up easy. My horse which wasn't quite a pony not quite a horse tied up on me so I couldn't give her anything but oats.
First of all though you have to know why he is underweight. If he is grazing all the time (ponies are really thrifty)and doesn't gain enough he probably needs worming, and I definitly would vet check him. HOrses and ponies need to graze most of the time on grasses, hay and high roughage stuff.
feeding once a day without grazing is unhealthy for them. You need to figure out his health status first before putting weight on him. Now if you are talking muscle weight, then let him graze all he wants on hay and watch fresh grass as ponies tend to be sensitive to to much grass, especially the first spring grass, let him graze about 5 hours a day if the grass is rich lush,if you can break up that grazing on lush grass that is even better, but let him eat all he wants if it is pretty rough. plus all the hay he wants if he can't get enough from grass and exercise exercise and exercise to build up muscles. ONly graze him more if he starts to lose weight.
hope this helps. I know that if you exercise him an hour a day such as on the lunge or taking him for a walk every day that will build muscles without tiring him our too much. I mean we can walk dogs why not ponies we can't ride if welsh b and c but welsh cobs can be ridden. By the way I think little welsh ponies are really cute! of course you didn't say what kind you have but this advice pretty much applies to all of the subtypes.
RMR