K.S.
#1, i know it is difficult but try not to compare! each baby will progress at their own pace
#2 not all babies crawl, some go straight to standing and walking
#3, if it makes you feel any better, my first son did everything at his own pace, so much slower than my friends kids... he did not crawl until almost 10 months (he hated tummy time, he actually crawled backwards before forwards), but then he promptly progressed to walking by one year. trust me, it is a totally different world when they become mobile
#4, to make you feel even more comfortable, i have 6 month old twins... neither are anywhere near crawling, neither roll on a regular basis and both are pretty content to just hang out on their backs and looking around.
just relish in the fact that you can still catch him easily!
for tummy time, i lay on my back and put my babies belly down on me, so that have to look up a little to look at me, they love it and it has helped them build their muscles.
good luck!