wow, what a story. you sound like such a dedicated and devoted mother. it really does sound like he's happy and healthy. i think you just need some reassurance on the choices you've made. before you change what you're doing or decide to supplement, make sure it truly is what you want to do. get all the information you can for both the risks and the benefits of continuing to breastfeed with or without supplementation.
i don't think you'd be happy with yourself if you make a decision selling yourself short of information. and if you can, switch to a pediatrician that is more supportive and knowledgeable about breastfeeding. often it isn't that they don't think that breastfeeding is good enough, it's just that they don't have all the information either. the lack of practice about breastfeeding in this country has really been influenced by formula use.
don't get me wrong, formula can save many lives when medically necessary. but really, how many babies medically need formula compared to all the normal babies.
here are some websites to give you reassurance and may answer some of your questions.
drjacknewman.com
normalfed.com
and some articles
slow weight gain following early good weight gain
http://drjacknewman.com/help/Slow%20Weight%20Gain%20Follo...
understanding sleeping and eating patterns
http://drjaygordon.com/development/ap/sleep.asp
good luck!