You can have him crawl over your legs to get a toy he really likes! This encourages him to be on hands and knees to climb/crawl over.
You can encourage him on his knees but also just encourage him and let him grow! Don’t need to rush him! Our baby girl only army crawls (and if that’s all she does, that’s normal too!)
One of my twins is the same way she’s in physical therapy for it rn. The therapist says it looks like she needs to build her upper arm strength cuz she’ll climb up on stuff and stand and sit up by herself etc but it’s just crawling she won’t do right 😭
Is he trying to crawl without pants? Pants can make it a little slippery. Also, lots of tall kneeling play really seemed to help my baby.