J.M.
My son broke his ankle at 12 months. No idea how (how's that for a terrible mother!). Only knew something was wrong because he wouldn't put any weight on it and cried when we changed his diaper (all that yanking on the ankle to lift his leg. Did I mention it made me feel like a terrible mother?). Bring your son to the doctor tomorrow morning. If they're worried they'll do x-rays.
The upside is that my son, while in a cast for 3 weeks, was completely unhindered, picked up walking again right after the cast came off, and now can run a lot faster than me (bummer). I'm sure your son will be fine, but get him checked just in case.