Well I'm glad to see that the colt looks better than I first imagined... though, ofcourse, not as good as he should
Since all of the horses on the place are wormy (unless you can get manure from your mares and put it in there?) my personal recomendation would be to to ask the vet about a probiotic paste or suppliment. If they agree (and I don't really see why they wouldn't, its fairly inexpensive) I used Probios Probiotic gel (Probios Probiotic Gel - Horse.com
) that I got at our tack shop for $6.99, but I use it on my minis so it might be more cost defective for you to buy the powder. I'd probably try to get a good 15-20g down him for a starter dose, then try a maitenence dose of 5-10g.