9 month old development?

My son is 9 months old in a couple of days. He was 6 weeks prem. He has gastro oesophageal reflux disease which is quite severe but eats orally fine.

He can roll over and has been able to for about 6 weeks or so. He has just learned to sit up unaided for short periods at a time. He cannot yet crawl. I am not worried about this side of his development however. The problem is I am a little worried as he does not ‘babble’. He can say agoo and has been able to from a young age but he can’t say ma da la or any of these other sounds. He is not vocal at all except for his obvious screaming episodes.