I really don't think that a foal should be sweated or ran at all. It is of NO benefit to it, and can damage it's body. Stick him in the round pen for a little exercise, sure- but let him self exercise. Do not run him.
At this age you want him to have as much turn out, high protein feed, and free choice- high quality roughage as possible. No sweet feed. We're giving our five month old (orphaned at 3 months, so technically weaned early) as much alfalfa-timothy mixed hay as she wants to eat, as well as 4 pounds of Purina Equine Junior. Our almost-eight month old colt is eating the same thing but 7 pounds of grain.
Here is a very helpful research article on feeds for young horses and development, if you're interested. http://www.uky.edu/Ag/AnimalSciences/pubs/asc111.pdf