I know some children who just went straight to walking -- crawling just did not do it for them. Your son might be one of those. I agree you can check in with the doctor just to ease your mind, but its really not a problem if he does not crawl at this age-- its ok.
I know that all children develop at their own pace in their own time, but you may want to talk to his doctor and have him evaluated to put your mind at ease. If he has stood before, that's a good sign, I'd think. Just keep gently encouraging him.