Min, I know it's really hard and you have already been told this, but DON"T WORRY.

They all develop at their own pace. Some children never crawl but go straight to standing up. (In some cultures, crawling is considered animal-like, and parents feel very ashamed if they can't stop their kids crawling!) Some don't crawl until after they are a year old. Slow to crawl does not indicate any problem and a couple of years down the track nobody will be able to tell when he started crawling.

I had a similar experience with my first baby. As all his friends started rolling, crawling, standing up and walking, he just sat. Until he was 10 months old. He hated "tummy time" and no matter what I did he wouldn't stay on his tummy for more than a few minutes a day. I tried everything I could think of and agonised over what I could have done wrong. And of course, no matter how sincerely you believe that variation is natural and fine, we all secretly want to have the baby the does things early!

After months of me trying to "teach" him to roll onto his front, push up and crawl (as per the text books and what other bubs were doing), one day he just rocked forward from a sitting position and started crawling by himself. So all my trying to help him had been pointless anyway. Incidentally, he only learned to roll when he saw his sister (2 years younger) do it!

This is what I suggest you do:
(1) Repeat after me "Jackson is developing beautifully. He will crawl or walk when he is ready"
(2) By all means try to teach him as long as it's fun and no pressure. But if you don't feel like it, just let him do it himself - he will
(3) If he's slow at crawling, he's most likely fast at something else, like vocalising, making expressive faces, fine motor skills (manipulating things with his hands) etc. If you look at his overall development, it might turn out that he's not getting moving because he's concentrating on some other aspect right now. My son was a couple of months behind with crawling and walking, but way ahead with talking.
(4) If he gets to about 18 months and still makes no attempt at all to walk, crawl or bottom-shuffle, then you have my permission to get concerned and take him to a paediatrician.