I Have A 22 Yr Old Son Diagnosis Global Hypoxic

I have a 22 yr old son diagnosis global hypoxic insult during deliver he was full term baby 8lbs 4 0zs he has been spitting since a toddler, surgery was done and the silivary gland were placed further back to drain extra siliva that was causing him to spit , that did not help and today at the age of 22 he spits constantly, is this a medical problem? his doctor and other all seem to think it is a behaviour.
Some suggestions please.