Hi Carien,
I can't give any advice from first hand experience as my case was the other way around (my LG crawled early, but then just didn't seem interested in walking at all), but I have heard that boys take longer than girls. In my LG's class at creche there was a little boy who didn't crawl, he just rolled over till he got where he wanted to go, and he seems perfectly fine now. Walking, talking, potty training, all fine. I also remember another little boy from my mommy-and-baby exercise class, who wasn't crawling at 11 months. Nothing wrong with him either, he was just a very calm, patient little boy, not easily frustrated.
When last did you go for a checkup at your pediatrician? That would be my first stop, rather than the occupational therapist. It may not be anything other than him being "lazy" to crawl, or it may be tied to something else (not necessarily physical). A paed would be better equipped to diagnose if there is a problem, and he can refer you to an OT if necessary.