Please don't give him oats!
I've watched a couple experiments people have done with their herds, and for the most part they've found that if oats are given after 3 1/2 weeks old, they're a HUGE risk of enteritis. For people that elemenated them from the diet until 3 1/2 months old, found that there were NO cases of enteritis.
I think for the most part all you can do is give him pellets, timothy, and possibly alfalfa hay. I know my little 6 week old litter loves alfalfa as a treat. It's high in calories, and I think that's what this little guy could use right now. o.o;
I've also had pretty good luck with rat/mouse/rabbit milk replacer from Petsmart or petco. I raised one of my runts on a mix of about 1/2 milk replacer, and 1/2 private feedings with mama. To this day, he's still a good 2 ounces smaller than the rest, but he's doing amazing. (Not to mention, the best bun out of that litter!)