Advice please: runner duckling is lethargic

I just got a new brood of runners yesterday, hatched on 4/17. One of the pencilled runners seems really run down. It's not being bullied, but hasn't been eating or drinking as much as the others, and has been sleeping a lot. And while the others have grown noticeably in just a day, that one is very small. The little guy doesn't move to snuggle with the others, and hardly opens its eyes when startled.

I'm feeding them all a game bird starter crumble, and I gave them Sav-a-chick electrolyte water in addition to regular water for the first 24 hours. The brooder has temp zones ranging from 85-90, so they can move to whereever they're comfortable.

The pic shows him with one of the mallard runners of the same age; you can see how much bigger the mallard is. Any thoughts on what else I can do to help this little one along?