R. -
As I worked on this reply, I think I'd suggest trying the second portion (geared toward changing his nighttime sleep pattern first, and then this first (daytime) suggestion only if needed. His behavior could be all about getting himself to sleep.
I'll offer this - as it may be worth trying. When my son was closer to your own son's age, he started his first biting and I put him firmly down on the floor, said no biting (pinching) and walked away (this was before he could walk after me). If he cried and when I returned, I always said no biting and made eye contact before picking him back up for: the rest of the story, the feeding, whatever it was. If he did it again, the session with mommy was over in the same way. This was for biting behavior in the daytime.
I'd read that the time with you is much more important to the child than anything else, so it can be a very a clear message that some behavior is not acceptable.
Of course, you have to weigh it against your child's temperment, if it seems to be working or not, and of course the dreaded overuse!
I think it is VERY interesting that after you left your son alone for a few minutes, he did struggle into sleep, and because you mention that this seems to be happening at night, he may be setting up a pattern of getting to sleep and having trouble doing it. This may sound odd, by my son was very stimulated by the presence or myself or my husband, and any source of light when nighttime came. We tried the family bed and HE didn't go to sleep like he easily did when placed in his own bed and lights out. I think some kids just keep going when anything (including us) is there to bounce (or pinch) off of and keep themselves from the sleep they so need. So, maybe start experimenting with a different night time pattern exploration for him (and you). As in, if there is anything he does right before he starts to pinch, maybe that's the sign to look for, and put him down then and see if he nods off more easily.
Best of Luck,
A.