First off, GREAT FOR YOU FOR GIVING YOUR CHILD HUMAN MILK!!!
There is nothing that even comes close to being healthy for your child except milk designed esp for him, and it changes day to day depending on what HE needs.
A red flag to me is any doctor telling you your baby needs a certain amount of milk. First of all, how would they know this? Every baby grows at very different rates-some babies are going to be 6'5" some babies 4'10, all babies were not created equal (in size) so I think that doc is way off base. Plus, doctors know illness and how to help with illness using drugs, they do not know 'parenting' or how to mother a child-only YOU know that. Trust your mothering instinct, your God-given instinct, and feed your son how much he needs and when he is hungry/thirsty. That might be 4 ounces at 6 am and 6 ounces at 8 (maybe he was more thirsty than hungry at 6, you get the point). My point is, babies need mama milk when they are hungry AND thirsty, just like we head to the water when we need a drink, they head to mom (or moms milk pumped :)). Try to feed him as soon as he gets up in the am, even if you have to wake him and as soon as you get home. If you sit and just read to him and put him to the breast, you will find he will want to nurse, it's natures greatest comfort food!! :) I would def recommend at his very young age to at least nurse him once through the night, he is still very young and his stomach is emptying out often, so he isn't getting nutrition through the night and that might be causing the slow weight gain. Also make sure you are ONLY using weight charts for healthy breastfed babies, not formula-fed babies as formula fed babies get an unnatural amount of unhealthy fats and calories that is why they are usually heavier (and have a high correlation of obesity later in life).
About the constipation-that usually is totally related to formula use. Breastmilk is a natural laxative so it's very very unusual to have contipated babies who are only breastmilk-fed. Formula has cows-milk proteins and that is linked to all kinds of health issues, one biggee being constipation (see Kellymom.com because I bet the research is on that site). Now having said that...many breastfed babies can go for days with no BM then BAM, they have a massive poopy diaper. But, the difference is it's not true 'constipation' as the BM is not hard, it's just delayed. All 4 of my children had weeks where they would go only once a week or so, and I at first would worry but then they would have a huge, loose breastmilk-BM. If your child is hurting though, and only you know this, then that is most likely constipation or maybe even hurting from the cows milk (but please don't switch to soy formula, there are a ton of cancers related to the massive amount of processed soy, see Mothering.com and do a 'search' for research on this).
Some great links to bookmark-
www.kellymom.com
this above site has a TON of answers for moms like yourself-great moms helping their child have the proper start in life, GREAT JOB again!!!
Another great site is
www.mothering.com/discussions
there are tons of links on this site, another one to bookmark for future use.
Hang in there, it gets so easy and you won't believe the bond you and your son will have, nothing else like it. Plus he won't be nearly as sick as an artificially-fed baby and YOU are protecting yourself from breastcancer in the process.
:)