Honestly I'm not reading all 50 other replies but children double their birth weight by 6 months, in my case i had a 21 lbs 6 month old (hello chunky!)but it is fine for children to not gain any weight for awhile after that. They pack it all on and grow out but not really up, then they even out and get taller. Weight gain is NOT an indicator of nutrion and as long as he's eating and drinking normally then he's fine.
Also you're right he's not drinking enough. I would stop breasfeeding if you're concerned about how much milk you're putting out, you may be feeding him 10 oz a feeding and not knowing it, unfortunatly the only way to know is to stop all together and formula feed, other wise i'm sure he's doing just fine, but you should offer formula BEFORE a meal and breastfeed afterwards, not the other way around.
I have a 2 year old who weighs 27 lbs, which he was weighed for about a year. He was born at 8lbs, was 20 lbs by 7 months and only gained about a pound a month, he's perfectly fine I assure you, as are most children. If you are really concerned and the doctor isn't then get a second opinion, always !