So glad you changed tactics even though he didn't use the safe word(s).

I'm not sure what his vocabulary is like beyond growling,
but Lord knows in an intense moment the right word doesn't always
come springing from my lips. Complicated safe words like
"watermelon" are totally beyond me.

If he's having trouble using them, then perhaps when he
says "don't" or something of that nature, you can ask him
if he meant - whatever word you have for "slow down" or "stop".
Then have him repeat the word after you.
The easier to say and remember, the faster that word or words
will become a habit.

Best of luck in taking it to the next level.