If you haven't already, try teaching him how to blow bubbles. He needs to learn how to purse his lips and build that lip closure and jaw strength first. If the bubble blowing doesn't work, try blowing out a candle and keep moving the candle farther away from his face so that he continues to have to blow harder to put it out. 2 years old might be a little young but as many developmental milestones, each kid is very different and better ready to learn a new skill when it is more child-directed (so if he's not interested yet, don't push it since it might only frustrate him). Don't worry, he will "get it" someday. Being that at 2 years old he may not have the concept yet of when it is appropriate to spit or not, you may have some difficulty with spitting at other times then just toothbrushing....if you don't want to deal with this yet then hold off until he's 3+ so that the behavior training can coinside.
Hope this helps.
Sincerely, M.(Pediatric Occupational Therapist)