This sounds like what my DD has been doing for several months....she'll do the same face you described, clench her fists, shake a little bit, make a little groaning sound, then laugh, and once she did that a few times she'd start crying like it bothered her. It used to worry me a lot, I thought maybe something was wrong. I asked the doc about it and even a few other moms I know said that it's usually just something they do out of frustration, and a lot of times it gets strong reactions from the parents so the baby will continue to do it. At first though, it honestly scared the heck out of me!! She'll be a year on sunday, and she only does it on occasion now, and it's usually when we make the same face at her, she'll laugh then do her "hulk hands" as we call it :)

Is he trying to poop? lol, serious question though. My daughter started making the weirdest face when she was about 6 months whenever she pooped. I have a picture of it if you want to see it.

If he does it after he eats solids his little tummy might be feeling all funny and he might be reacting to that

I was thinking it is maybe his "thinking" face, you lnow how some people stick their tongue out a little to the side when they are really concentrating, and mybe the cries are because of frustration.... But poop sounds reasonable too.

My DD does this similar thing where she clenches her fists, sticks out her lower jaw and makes herself almost tremble - as if she's putting in all her strength to squeeze something with her fists. She never cries though. She actually laughs when she does it. We laugh too because it's so random and she's been doing it for months now. Have you asked your son's pediatrician about it? If you're concerned, you should definitely ask.