No offense, but FIRE YOUR PEDIATRICIAN! What is he thinking?! That your son should be drinking JUICE instead of BREASTMILK?! He is on crack! Ok, done. Listen to your intuition and your son. If he only likes breastmilk and water, consider yourself lucky!! And remember that weight averages today are much higher because of all the obese children, so if he's under the weight average, then you're probably right on track. My daughter is tall and skinny, just like her dad. She is still very active and happy, and has plenty of wet and poopy diapers (though she is potty training herself at the moment).
Breastfeeding your baby is the best thing you can do you for your child, the longer the better. If you can make it to 2 years, that is awesome! The World Health Organization recommends AT LEAST 2 years, and the worldwide average weaning age is closer to 3-4 years old. We have one of the worst breastfeeding records for a developed nation, and one of the highest infant mortality rates as well for a developed nation. Possibly a coincidence.
Anyways, if your son is having enough wet diapers and bowel movements, then I wouldn't worry about him getting enough liquids. My daughter was 19 lbs for months, it seems, but shot up in height. Now at 21 months, she just got to 23 lbs, but she's 34". Like I said, tall and skinny. Don't worry too much about weight unless there are other signs, like listlessness, or not many wet diapers. Keep on introducing the cup (I recommend Klean Kanteen - no toxid plastics). When he's ready, he'll go for it. I don't think my daughter got the hang of them until closer to a year. She just liked chewing on them, too.
Another GREAT support system is the La Leche League. Go to www.llli.org to find your local leader and to find out when they meet.
KEEP BREASTFEEDING!!! BREAST IS BEST!!!