I'm sorry to hear little Mooncake has hurt his leg. Young ferrets are especially prone to injuries like this as they can play quite exuberantly and do not have the strongest bones/muscles just yet.
You've done the right thing by stopping play and examining the leg for any obvious injuries or swellings. I would also ensure he is now in a small space that is nicely padded. Check that he is breathing normally (not too fast/laboured) and seems to be alright otherwise.
If nothing seems abnormal and the leg can be flexed and extended the same as the other, the best case scenario is that he has strained it and is tender. If this is the case, it should gradually become less painful over the next hour or two and he should begin to use it more and more.
However, there is always the chance that he has sustained a more serious injury such as a dislocation or fracture so if he continues to not use the limb, it is best to have him seen by a vet, who will likely want to do an x-ray.
I would strongly recommend seeking a vet with good knowledge of ferrets. An experienced ferret vet would be able to provide the option of limb surgery if required.
There are a couple of pain relief/anti-inflammatory medicines that are used for ferrets and can be prescribed by vets to help with any pain or swelling. These are useful whether it is a bad sprain/strain or a break.