W.B.
Hi A.,
I dont think you have anything to be concerned about, my 3rd child did the samething the only thing that would move was her wrist in circles, she could sit up and her arms would be fully extended out to the sides of her at shoulder height also she would just sit there sometimes excited sometimes not and like your son when she would have a toy she wouldn't do it.
My 4th child is getting to where you can walk around and put her on my hip but her arms are out to her sides. The do all kinds of silly things.
I watch an 8 month old that can army crawl like there is no tomorrow but he can't figure out how to crawl on his hands and knees. But he can get on his hands and stand on his feet, he is still trying to figure out what he can do.
With my 3rd daughter she sat there until she was 9 months old just looking around at all the kids playing and then one day she just took off like she had known all this time what to do bt she was just taking it all in.
The day will come when he no lnger sits where you put him and thens it's even more fun from here on out.
Hope this helps W..