When should baby's start crawling my son is 6 months and is only rocking back and forth

My son started scooting all over the place and pushing himself backwards at 7 months but officially began crawling as soon as he turned 9 months

My first born skipped crawling and walked at 9 months and my second crawled at 7-8 and walked at 11

OK thanks y'all my husband was getting worried and I told him I believe he is on track just wanted to double check thank y'all

9-12mos