Hello!
Children can get fever due to any injury or with severe stress such as fractures.
It can be an infection if there is any open injury; for any open injury you have to make sure that you cover it with proper antibiotics and also give a tetanus shot.
Get him examined and check if his cast is properly put and give him paracetamol or acetaminophen every 4 hours till the fever subsides.
Take care!
I hope this is a moot point and you have already taken him to be checked, but I also aree a visit to the ER is a good idea.
It does sound like an infection, but it could also be an allergic reaction to something they use to make the casting material.
Definitely better to have it checked out sooner rather than later.
Sounds like an intection, did he have surgery or just the cast? I would go to the ER right away just to be sure everything is okay. Infections can be very dangerous! Good luck and I hope everything is okay!