S.Y.
Like someone else said... every baby is different.
My son didn't roll over till 8 months (did it constantly for a week... the novelty must have worn off and he barely did it after that until much older)... he didn't crawl (not in the traditional way on all 4's) till 12 months -- he was mobile in his own goofy way though. He didn't walk until 16 months.
I wouldn't worry too much. Like your ped said... he'll do it soon enough. Just keep doing what your doing... if he gets mad enough on his belly when he does get rolled over - he'll roll back I bet :D I would just keep putting things just out of reach... just enough that when he stretches to grab it, he forces a roll. Not to worry though, he'll get it and then you'll be wondering why you wanted him to get moving soo fast - LOL once he starts he'll never stop :D