Poor guy. Rest is going to be the main thing. I would also consider an anti-inflammatory drug. I use ibuprofen and I know others use aspirin. Here is the dosage for Infant Motrin - 5-10 mg/kg 2-3 times a day. Or 0.1ml per lb. Just don't use grape flavor. I heard there is something bad in grape flavored drugs, so I always avoid it just in case.
I would also feel over his foot and compare to the good one. See if you can feel any differences. If you feel anything different from the two, it is likely just sprained.
Giving him water therapy can also help take the weight off his leg and allow it to heal and strengthen with less weight on it. Deep water where he can't touch the ground is the best. This is especially good with Pekin, because they are so much heavier and take a longer to heal.
It is always possible that it is infected too, you can have an antibiotic on hand if things worsen. Otherwise, I would try other things before an antibiotic.