Alot of babies don't crawl. Some will skip right to walking. Crawling isn't even considered an official "milestone" for lots of babies.
More importantly, can he go easily from lying to sitting positions, pick up tiny objects with his thumb and forefinger, possibly pull up to his knees or feet, roll around easily, etc.
My neigbors son never crawled and didn't walk til 15 months. Now he's 2 and just as active and coordinated as any 2 year old.
I remember the pediatrician told her that the biggest thing to notice is if you put him on the floor on his tummy, can he get into a sitting position. If not, he may need to be examined for muscle tone or other possible problems. Of course, if it's really concerning to you, discuss it with your pediatrician anyway. He/she should be able to ease your mind!
Good luck!