I think that your pediatrician may be right about this one. My son lay on his back when he slept at that age, and so the back of his head seemed a little flat, not to mention the loss of hair. He looked like Friar Tuck for about 5 months, but when he started rolling around, changing positions when he slept, and sitting up, his head evened(sic?)out and his hair grew back. I guess it's different for every baby though.
You might want to try a positioner http://www.amazon.com/b?ie=UTF8&node=166874011 if you're really concerned, although all the experts say that using these can increase the risk of SIDS. I had to keep in mind that he still had a soft spot and so his skull wasn't completely formed yet, so it will change in time. I mean, you have at least until 12months before the Fontanelle closes up.