Can I have a separate car insurance policy for my son on my car?

It usually works out cheaper to just add another driver on the same car insurance policy. If you're concerned about your NCB, you can get protected no claims discount which means that if your son is involved in an accident where he's at fault, your no claims discount will not go down.

However if you think it's still worth him getting his own insurance policy, then you can do that as well. Many insurers though will frown upon the policyholder not being the registered keeper (I'm assuming you're the registered keeper here) and will not provide cover if that's the case.