I'd have to disagree with that. I have two other TB's that are doing perfectly fine on two flakes a day, with no grass. He lost wait due to dehydration. Also with the barn i'm at, he can ONLY get fed twice a day.
Now if your horse is underweight, you need to get to the real root of the problem. It would be a teeth issue, it could be inadequate feed, it could be parasites.Have you had the Vet out to look at the horse?
Regardless, as others have posted you cannot "quickly" add weight onto a horse without there being adverse effects-like colic or founder. If you look around the forum here, there are various threads on how to correctly bring a horse back into condition without overfeeding.