We might actually pop our M&G cherry on this one too
Posting as the male of this partnership, I can kinda understand why he was so torn up. My partner and I , back in the day, used to roleplay over txt message that she was getting with someone (she wouldn't be, but we were testing the water, so it was semi-realistic). I'd get a little hurt when something we'd agreed upon beforehand didn't pan out, such as texting before it "happened". In this case, I'd say that in your partner's mind, forgetting to text him beforehand was akin to forgetting about him, which would be very hurtful and possibly catastrophic in such an emotionally sensitive circumstance.
On a macro-level however, if this was really his idea, I'd say he should have done some more thinking about how he'd feel (maybe have run some "practice" scenarios ) and made sure you understood how important it was to him for you to keep in contact with him. Judging by your post, you don't understand why he was so upset, which is pretty indicative of a failure to communicate.