Can normal chickens get up if on their back postion??
I think so. You could go lay a hen on her back and see

I read a thread a while ago, where someone lost a rooster that had gotten stuck on his back and couldn't get up-- apparently it's common with some really wide birds (show-quality Cornish). I wouldn't expect your rooster to have the same body build, so I don't know why yours is doing it.
Here's a link to the post that was talking about it:
https://www.backyardchickens.com/th...r-meat-and-more.1069243/page-11#post-18200432