34 weeker

My son was born at 34 weeks, he'll be 6 months on April 27th and he's nowhere near sitting up on his own OR he can't roll from his back to stomach. However he has been rolling from stomach to back since 4 months and he can completely hold his head up and bear his weight on his legs when I stand him up.
Could he be a little delayed with sitting up due to him being born a month and a half early? Or is there any other 6 month olds sitting up?