I have a 15yr old son who satarted complaining of foot pain on Saturday. He says he never injured it but hurts when he walks. He does have a partial torn ligament in left ankle so I assumed that's what it was. He says his ankle doesn't hurt but under his foot does. I noticed Monday he has a lot of swelling on inside his foot from toes to heel. What would cause this and what should my next steps be?

Next step -- go to a doctor.

I realize that and going to take him asap but what would cause this? Never heard of this especially in someone so young.

There could be an internal infection, a displaced bone, a messed-up muscle or tendon -- a doctor will figure it out.