Yes! That's like asking 'my son murdered someone, but when the police showed up he wasn't holding the weapon, can he still be convicted of murder?' doesn't matter if he was in or out of the car when the police arrived. point is he was driving while drunk.
It's responsibility of parent/s to inculcate good values in their children and bring them up as honest citizens. Drinking at age 19 is worrisome , making accident under DUI is troublesome and fleeing the scene is unpardonable . Having made an accident under DUI , he should have called the cops voluntarily & given honest statement. Hitting a Tree is as bad as hitting a human being as both grow each day ! One MUST not look for advise or means to protect an offender !!
Of course he can be charged, not sure in your state how strict the law is there, but if I'm right they could also charge him with fleeing and can and may put on a warrant for your sons arrest, I would hire an attorney immediately.