Separate them. If they are both yearlings they need to be separated anyway until the stud is gelded. Otherwise you might have quite a mess on your hands and it won't be related to a chewed up mane.
Babies tend to chew on things (a lot of boodmares end up with half a tail.... ;) ). You can try products that are designed to stop chewing because they taste bad, but you have to be careful that they won't harm the horse you are applying them to (irritate the skin) and you might have to experiment to find something that works for your colt. Of course this is in case this continues after your colt has been gelded.