Sounds like he is stimming. He could be finding his own way (the clenching and squeezing) as a way to regulate himself. I would however talk to your ped and the dentist about the teeth grinding. They do may things he can chew on, but they may have another solution for the grinding.

Sounds like stim or a tic. It could be a sensory thing, he just likes the way it feels. It doesn't sound like its something that would be particularly disruptive so personally I wouldn't worry about it. The teeth grinding thing is more worrisome since it affects his teeth. I've never heard of anything you can do for grinding except wearing a night guard while sleeping but it would be worth asking his dentist.

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Get a mouth guard or Chewlery for the teeth clenching.. Is sounds exactly like stimming! It is a way for them to calm themselves when they feel anxious. I have not really ears that what you explained had anything to do with seizures. My 6 year old PDD/NOS grinds in his sleep, jumps, flaps (hands and feet) , hums, and has facial ticks that are from his Tourette's. I would seek an OT for a Sensory evaluation. We found that feeding his sensory needs helps him to control these things!

He could just be using his fingers like a baby uses a teether to help with the discomfort of the teeth coming in. Getting him something else he would like to chew is a priority because mouthing his fingers could turn into nail biting which is terrible for your enamel and one often described as a harder habit to kick than smoking. Also having hand in the mouth all the time is like licking every single thing the you touch or pick up. Kind of gross :(