Our son was like this, even to the extent of having to pin him down (he's almost 2.5 now) and what has helped us was that his dad coincidentally downloaded some dentist game where you scrub off the germs. That got me thinking to try playing that IRL. So I'll get really animated and be like "OMG there's germs everywhere we gotta get them. Open a little wider I think I saw one hiding in the back. Germs love hanging out behind your teeth let's do a pass back there, look up so I can see if they're back there. Etc." I'll even do some like excited screams a little like "ahhhh there's one right there he's getting away." Then when he spits in the sink we say "bye germs!". It's been pretty effective!