I would have thought death grip syndrome too, until you mentioned his reaction when you got him close that one time.
I have a feeling...a gut instinct, telling me that maybe there is some psychological issue he has about orgasm. Maybe he thinks it is dirty, or thinks it is rude to cum on/in you. Like its shameful? Maybe he is extremely embarrassed of his cum-face, or what his cum tastes or smells like.
I mean....if you got him close and he stopped you, this suggests he CAN cum...or can at least get close enough to panick that he was about too....but then he stopped you.
A man with death grip syndrome would probably not stop you. If anything, this man would probably be so happy that you made him cum! The fact he pulled away honestly suggests he has some fear or embarrassment about cumming in front of you.
For this reason, he has probably trained himself to hold back and never cum with you. I mean, aside from the time YOU had control and he almost lost his.
Yeah it does sound like there is something he is worrying/concerned about and I suggest talking openly and honestly but going very gentle. If he is embarrassed or ashamed, he will be defensive if you try to probe. I suggest not talking in the bedroom, but outside of the bedroom and maybe just asking questions gently, probing to see if he offers up what is on his mind. If he has some deep seated fear or phobia, it could be much harder.
Say to him..."The truth is, it feels amazing when you give me pleasure and orgasms and I know you say you are different from other men and don't need them, but I have to say, this is hurting me and making me feel a little insecure, because I want to give you this pleasure and the fact I can't makes me feel useless/not good enough. I would just like to understand why you don't orgasm with me. If you are worried about something maybe?"
If he says "I just don't" or closes up and gets defensive, you need to reassure him it is not an attack, but that you are feeling hurt and it would help ease your hurt and worry if he could have the strength to help you understand what he feels.
I mean...There are people (men included) who have never had an orgasm and still are happy and content to enjoy sex in all its forms, but what pricked up my ears was the mention that he almost did cum, then stopped you and jumped up. This tells me he can potentially cum, but doesnt want to (It was a deliberate attempt to avoid orgasm)...I also notice you said it was during a blowjob, when you had control. I have a feeling I don't have to ask this, but does he often take control? Like, control the thrusting, prefer positions where he can control it? If you ask to give him a blowjob, or go on top, does he try to persuade you into some other way?
I dunno....just a gut instinct that something is bothering him about his own orgasms. I guess this is what you need to figure out.
Good luck x