Hi T.,
Oh my gosh, I think your doctor is alittle off base here. Babies develop at different rates. Some want to crawl/stand/walk very early in their life. Others are quite content with just lying around. At this age, they probably can barely sit up on their own, let alone roll-over. Your babies were born a little early, so they probably won't develop at the same rate as full term babies - their a month behind (at the very least). Acknowledge their differences in developement - they are two different children - they'll never do the same things at the same time. My children are now 16 an 12, but when they were babies, my older one, didn't stand until he was 8 or 9 months, my younger one was walking around furniture at 5 months - different kids - different development. Some babies never crawl, they go straight to walking, some crawl/skooch until they're one. Putting your baby in an exercauser will be fine, but relax - he'll get there! If he was 9 or 10 months and showed no interest, then I'd worry. But right now, relax and enjoy him.
Congratulations!
C.
By the way, I am a mother just like you. I own developmental toy business. I have no "degree"in the medical field, but because of information from the company I am involved in, I have gained lots of knowledge and experience in child devolopment.