Crawling...or not

For those of you moms out there whose kids did things early or right on time please try to understand how it feels when you're kid does some things late.
Ethan sat up on time, rolled over on time, and started babbling early. He sat up at 6 1/2 months and sat up really well at 8 months. He's been scooting backwards for the last 3 weeks (when he was 9 months). He just doesn't seem to care to crawl. He's 10 months old and I have begun to worry. The books say that it's not uncommon for kids to just get up and walk and skip crawling all together or for them to crawl later than 10 months old. I feel like I'm placing so much expectation on Ethan to keep up with my friends kids. I keep forgetting that he's an individual and the books are just averages. You always want your kid to be more than average. I have friends whose kids crawled at 6 1/2 months or walked at 10 months and my kid just doesn't care to do it.
I wonder how I can let it go when in the back of my mind I'm worried that he'll never do it. But, then I remember that there was a time that I didn't think he's ever roll over, or sit up, etc. He did all of them and he did them in his own timing. I've got to just let go and let him be Ethan.