Is he happy and healthy? Some children start to become the body shape and style they are meant to be at this stage.
Honestly, my guess is that he's dropping weight because you are adding rice cereal - which does not have much nutrition at all and has no where near the fat and calories of breastmilk. He's eating rice cereal instead of taking in breastmilk at that time. YOu've taken away a big chunk of his calories and fat. Even if he eats after the cereal, you've filled his tummy with something less nutritious and he doesn't have as much room for the good breastmilk.
Pediatricians do not know that much about breastmilk, it seems. They recommend starting cereal at 4 or 5 months when the American Academy of Pediatrics suggests waiting until 6 months.
Another HUGE thing to consider is that the growth charts were developed based on FORMULA FED babies. It's totally different for breastfed babies. Ever notice those growth charts and "recommended feeding schedules" on the walls at the pedi's office are always sponsored by Enfamil or Similac? Not a coincidence. This is a GREAT resource for information (solid, research based information from a certified lactation consultant, IBCLC) on feeding your breastfed baby solids: http://www.kellymom.com/nutrition/solids/index.html
If you still want to give him cereal twice a day, make sure you nurse him to his fill FIRST and then give cereal afterward. It's really only for practice, since ounce for ounce the fat and calories of cereal can not equal that of breastmilk.
Oh, and breastfed babies at 4 and 5 months old don't usually sleep through the night. Keep in mind, too, that "sleeping through the night" at this age is considered 5 hours at a time.
My suggestion would be to check out if he seems at risk to YOU. Rolls don't mean extra health. I think some questions to ask your self: Is he happy and content after eating? Is he hitting other developmental milestones?
My friend and I have had our last babies within a couple of months of each other. Her girl was born a couple of ounces heavier and the same length as our little boy. Her little girl fell off the growth chart pretty soon. At 6 months she is 14 lbs and 25 inches, while at 4 months my boy is 17 lbs and 25 inches. We use the same pediatrician. He is not worried about EITHER of our babies. Babies start to grow in the pattern they are meant to be.
It sounds to me like your biggest problem is a pediatrician who doesn't support breastfeeding much.
Good luck. I really don't think the issue is your baby's growth, but more with a pedi who doesn't understand breastmilk. Please don't jump to supplement with formula till you've really looked at everything UNLESS that's something you WANT to do. It does NOT sound like something you NEED to do. Big hugs - breastfeeding is still vastly misunderstood by the pediatricians in this country. Formula is NOT the same nor is it "just as good". Formula companies even tell you "breast is best". If that's your choice, it's fine, but I think a 5 month old 15 lb boy who is thriving in every way does not need extra formula.