Ehh I know whatcha mean. I'm hesitant to say this directly applies to your friend, because the SSRI comment makes a lot of sense too.

But anyway, I'm like your friend. Rather I was at some point. Or am sometimes? I don't know. But I had a huge ego. I was a righteous jackass. Not a loud one, either, I've always been pretty introverted. I absolutely could not stand to be not "in control." That made letting go and spiraling into a trip next to impossible. To put it plainly, I was mentally ill (who isn't, to some extent?). Like I said, not necessarily true of your friend, but maybe.

For a particular and pertinent example, one time a group of friends and I took 5 g's each. Nobody could talk or do anything except me... and I could act and communicate perfectly normally and direct the group around as necessary (and it was necessary... had to chase a particular friend down a few times.) I even called one of our sober friends over to help manage the group and hung out and spoke with him a lot. I was also still definitely tripping... In fact the mushrooms chose that trip to really press their influence on me. Their ripping at my ego was so great and my clinging so tight that I had several nervous/terrified breakdowns, though you couldn't tell from looking at me. I'm good at turning into a stone, you could say. Eventually that ripping and tearing became undeniable, but I still could not let go into them, and I believe this is how I developed schizophrenia. My internal monologue, already monstrous before this trip, became identified as "not me." It screamed and raged at me during that trip, while I acted totally "normal." I did not have the capacity to show that weakness, to be vulnerable, to/with anyone.

Trips and months pass, I eventually stumble into Tool's music. I trip to the album Aenima at a particularly healthy, fit, happy point in my life (dropped out of school, working in physical labor) on only 2.5 g's. It destroyed that ego. I was totally reborn. Here I am today, and I have no problem descending into a trip. My mind isn't quite fixed from the schism I ripped into it, but it's much, much better. Much better than it was before it split into schizophrenia, even. I'm much more open to the spiritual interpretation of things now, and no longer trivialize my experiences as mere "hallucination" like I once did. All the same, when last I took DMT, even though it was a lot, I couldn't break through. It's important for me to be alone to "let go" is part of it. I just don't feel safe enough around most other people (though a few I do fine around.) Past experience, parents, neurosis I haven't overcome yet, etc.

So maybe your friend is like me and just has a really, really thick shell to crack. But you certainly can't and shouldn't push him to crack it. It nearly broke me. It isn't to be rushed. If he is meant to overcome it, it will come in its own time, and it will be beautiful no matter what darkness he crosses to reach it. I also caution you, if he's like me, from tripping with him. I do think I caused undue stress in some situations. Plus, clinging to your ego, fear, and the emptiness in the face of such potential for love and growth can make one a little... cold. Nothing to be afraid of, certainly still to be treated with love and patience.

I hope my experiences help shed some light on yours. If not, I hope they were interesting. ;)