People telling him to stay away from the acid-space are basing that in FEAR. Fear is the root of any bad-trip, and it really doesn't even sound like his trip was that bad... more like out of control. the psychic damage was probably minimal.. i mean, at least he didn't think he was dead, or try to jump out of his own head or anything.
If anything, an extremely powerful acid experience, once integrated (which can take years), will make it that much easier to control powerful experiences in the future. Actually, control is the problem.. once you realize where acid can take you, you will find that letting go, just being is the key. But still, being able to do that is learning. If you run from the experience in fear, you've learned nothing. Everything is worth something.