My 7 mo old often shows a brief jittery move (~2 sec) where he clenches his teech, straightens his arms and legs with a little tremor. is this normal?

Sounds like a shuder: Some infants & toddlers will have a brief set of movements as you describe that are called shudder attacks. If brief & kid seems aware & responsive to you during and after it is probably something you can monitor & report to the babies dr. If they appear to be coming more often & of greater intensity i'd have him seen in the office.