Just found out my son had a tendon wrapped around his perennial nerve after a nerve transfer also some scar tissue will that impair movement in foot?

It can. If the nerve is damaged or entrapped it may lead to numbness or weakness. Have it evaluated by a professional.
Peroneal Nerve. Definitely can impact the nerve if it is under compression and may there for effect the ability of the foot to pull up and pull to the outside. The peroneal nerve has 2 branches typically that control the muscles on the front and outside of leg the controls movement of foot and ankle. Why did he have the nerve transfer?
Confused. It's unclear to me what injury or what surgery at your son had. Your description does not bring to mind anything familiar. It sounds fairly complicated so i would recommend that you discuss this with your son more or obtain a doctor's consultation. Sorry we can't be of more help.