Is my baby developing properly?

I hate to repeat what your mum says but shes right - all babies do stuff at different times. My son is 9 months and has been crawling, rolling, sitting etc for about 2 months, my friends little boy is 12 months, still doesnt crawl or roll or lay on his belly, but can take a few steps. My son started crawling then pulling himself up on furniture and easing himself back down without falling all in a matter of days. You might find that once he is confident he can do it, he will just fly! Still get him checked out to ease your own peace of mind, but try not to worry too much (I know thats easier said than done - worrying unnecessarily is part and parcel of being a mum!!)