9.5 month old still not crawling

My son is 9.5 months old and pivots around on his belly or army crawls (kind of leads with his head), but no actual crawling on knees. I try placing him on all 4’s and supporting his knees/hips, but he just gets upset and gives out immediately. He can sit up unassisted if I place him there but can’t put himself in the sitting position on his own. My older daughter was moving around holding onto furniture at this point so this is different for me. Should I be concerned about his core strength? Any suggestions to help improve?

Hi, thank you for reaching out. Can he get out of sitting? I would work on transitions in and out of sitting. I would also work on playing at a small height like a stool or an activity table that is on an angle to start strengthening the hips. Its easier than hands and knees. Have you tried putting him partially on your legs and having a toy on the other side so he will try to go forward and get it?
I’m going to attach our developmental milestone round up which has tons of blogs as well as our youtube playlists by skill and our 6 week programs if any of those are helpful for you! Here are some videos I did of a kid who was really good at sitting but wasn’t doing much other movement as well:

He can roll out of sitting to his belly, just not moving from floor to sitting position. Thank you for all your resources, I will definitely try them out!