7 Month old Hand Twirling

I have a 7 month old boy. Everything about him is perfect and normal.  Laughs, smiles, blabs, loves standing, jumping, and loves to cuddle.  Around 4.5 months I've noticed that he twirls his right hand and his right foot.  The foot not so much, but the right hand in constantly in motion.  When he is calm and relaxed such as in my arms, or riding in his stroller the movement is gone.  When he wants something or when he is anxious, or maybe does not like something the hand starts to twirl.  Its noticable and beginning to worry me.  My first son never had this nor did I ever hear of this.  At 6 month check up his doctor said that he is delayed in motor skills because he was not sitting up yet or showed any interest in it at all.  Plus the hand twirling.  She said she has seen babies who twirl their hands and were delayed in motor skills.  About 1 week after that visit he began to sit up on his own.  Now he can sit for minutes on his own, once distracted he may tumble over.  His sitting is not perfect but I guess is ok being that he is only turning 7 months in 2 days.  When he sits he does not really use the right (twirl) hand to maintain balance and that I think may cause him to tumble over at times.  I tried doing research online on any info on infant hand twirling and cannot find any piece of info.  If someone can help me with any info this it would be much help for me.  I am thinking of making another appointment with his PI and possibly a pediatric neurologist.
Thank You