My 7 month old can crawl but can’t sit up

My son turned 7 months on 02/05. He’s been army crawling since 5 ½ months & recently he’s been getting up on all 4’s & rocking like he’s gonna start traditional crawling… I’ve been working with him on sitting up but he has zero interest. He just kinda rag dolls himself & flops over instantly whenever I put him in a sitting position or he’ll cry until he’s taken out of the position either by me or flopping himself over lol.

Is this normal?? I know babies do things on their own time, this is my 3rd baby but my other 2 were sitting independently at 5 & 6 months.