Corkers sounds like he's getting close now
I would encourage some cross pattern...you could try simply touching hands to opposite knee, opposite ears, opposite feet...I don't know if it will help him crawl but it's good for cross patterning in older children. You could also gently encourage the opposite movement of knees while he is attempting to crawl, being very careful not to have baby fall flat on face, as usually the legs are stronger than the arms.
Most kids sit before they crawl or very close to the same time, often they actuality crawl because they were sitting and fell forward to reach something! Also kids who don't crawl and head strait to walking have been known to have coordination issues later so it's goo to crawl for a while.
As an indication of what can be done only, I really didn't try that hard! obviously some kids will take longer for many different reasons. Bulky clothing, colder weather, all carpeted floors or all smooth floors, furniture placement, time and opportunity spent at home, natural preferences And teperment( those ones that cry during tummy time! Yep I had one!) siblings, safety...endless list really. My kids rolled before they had their first lot of needles ( scared the crap out of the nurse, who turned away while bub was on the bed!), were all sitting before 4 months, crawled around 6 months and walked at 8 to 10 months. They were little but tall kids. Breastfed and not chubby, I think all chubby kids will take a bit longer to defy gravity. And their is nothing wrong with a chubby baby
I believe my early movers were a result of increased opportunity to move. I never swaddled, and almost always put the on the ground, I regularly did my activities on the floor with them. Folding washing, reading etc
After they learnt to walk I actively taught them a cross pattern run, I even took them jogging a few times to build up persistence and fitness. And yep they were much fitter than me!
We used all the climbing equipment at the park, jumped on the bed
, and I taught them to summersault at 1 ( do be sure they have strong necks before you attempt this and get instructions!) peddling bikes and swimming came next and I wish I had done more ball skills with the girls. use a balloon to start with rather than a ball.
All kids will develop at their own pace yes! But don't let that stop you speeding it up