All illegal drugs are bad but Ketamine is class B. I’d be worried about it as a gateway drug and also if your son is on it for a long stretch he may get a real tolerance for it. It gets used in conjunction with coke, which would be very bad because cocaine is so addictive.
I’m not sure if that helps. Something I do regret is that I told my son all drugs were very bad. I wish now that I had really driven home what heroin does to someone. You don’t want your son using a party drug like ketamin but I think you need to show you know a bit about what your talking about so that they don’t just dismiss advice as mum on a rant about ‘drugs.’