If your child is pretty good with competition (this won't work with kids who are sensitive about not winning things or who don't want to participate in competition) Have a spitting contest. I know, as you said, "not a behavior you really want most of the time" BUT there are times for it - you've found one.

Practice spitting watermelon seeds and things that do NOT taste good like his toothpaste probably does (explain this behavior is only for really special times like in the bathroom and adult condoned spitting contests).

Have a family contest in the backyard.

Then, when you know he CAN spit (because now he has gotten help from you, siblings neighbors etc and shown you he's pretty good at it right?), you can use water swishing to help him have enough "stuff" to spit when brushing his teeth - just don't use so much you negate any effects of ongoing protection.

You might also try making a game out of it like, how long can you brush without swallowing OR spitting? Brush together and see who "wins". If you aren't having a contest, just encourage him to spit more often than you would yourself, use an egg timer to have him spit every 30 seconds or so. . . Make it fun and silly, he'll laugh, you'll laugh and maybe he'll get it.