Is it normal for child to stiffen arms while hand movement?

Hi doctor,

My son is 7 months old. A few weeks ago I have started to notice some odd movement, like opening and closing his hands and mainly on the right hand (just like stimming). And when he was lying on his back sometime he will stiffen his arms and legs and grunts. My partner's aunt's (his mum's sister) son has been diagnosed with autism and I do not know whether that will put my son at a higher risk.

Hi doctor,

Thank you for your reply. I would like to know if the stiffening on his arms and legs indicate anything like GERD or something because he does it every day and when he does it he will stiffen for a few seconds and relax then do it again and again until I go distract him. Also, he spits very little amount of sour smelling milk quite often too.

Thank you doctor,

One more thing, does the constant hand scratching like motion typical of baby like him? At first, I saw him scratching actual objects but recently I noticed that he does it with his arm in the air too. It is just like the one in this video but sometimes more obvious then that just like the motion from the beginning of the previous video.

Hi doctor,

Thank you very much.