Hi my LO is 7 months old.. but not sitting up yet on his own and barely even when he is propped up. He's very active on his belly and back. He is never still, always in motion. If I try to sit him up, he immediately wants to stretch out his legs and stand but obviously he can't stand yet. Also, he does this thing where he lifts his arms and legs at the same time and moves them real quick like he's swimming. Should I be worried or concerned that he isn't sitting up yet..????