Well, it's not just like a quick shudder it can go on for minutes. It's not really noticable but I notice it. And it's almost the same pattern as breathing (breathe in, pause, breath out but it's shiver, pause, then shiver again). Sometimes it doesn't look like shivering, but yet tightening the muscles then relaxing... almost like when people do crunches.
Maybe he's working out? LoL I don't know... I just want to know if it's something worth taking him to the doctor for...