You mention so much here, that I'm gonna try to take it in the order you mention it. Sorry in advance, because I know this is gonna be long :-)
First, I second pretty much all of what Amber said.
Twice a day is so far from being enough.
Hydration: The signs of dehydration that you listed have me a bit alarmed, honestly. If it were me, I would completely eliminate solids until I was absolutely certain that my baby was well hydrated again. Hard stools may also be a sign that your individual baby's digestive tract is not yet ready for solids. I know the AAP says 6mos. La Leche League and the WHO both say "middle of the first year". From 6-12mos of age solids are for exposure to tastes and textures and are not intended to provide much, if any, nutritional value.
Also, I would add a bottle of water for those in between times when he doesn't seem to want the breast. Watch his diapers. How many wet/dirty diapers in a day? Most solely bf'd 6mos olds that I've known were having 5-6 wet ones with a dirty every 24-48hrs.
*If this area doesn't improve quite a bit in the next day or so, I'd be calling my pediatrician.
Pumping: Not all women's bodies let down well for pumps. 4-5oz is the most I ever get, even with a baby nursing on the opposite side! With #1 I felt lucky to get even 1 whole ounce! :-) My #1 was 25lbs at 8mos (right before we intro'd solids). My #2 was 20lbs at 4mos! So I knew my babies were getting enough :-) Bottom line here: Just because you pump 3-5oz, don't think that's all he's getting. At 6mos, I have no doubt that he's a very efficient nurser!
He's started to scream at the breast: If it seems to happen only at "sleep" times, I would suggest removing yourself to a quiet room with as few distractions as possible. I realize that you have a 4yr old as well. Perhaps sit on the couch or bed and read a book together.
At night, I lay down in the dark with my 14mos old to help him realize that his only options are bf-ing and sleeping.
Also, I know some babies (mine being two of them) that seem to have a need to release some pent up energy before falling asleep. Some scream, some cry, some just moan or sing (my two). For me this started around 4-5mos of age. Just a thought.
Supply: Your milk supply might very well be dropping simply because he is nursing less. Breast milk is a Demand and Supply thing. The baby Demands more by nursing/suckling more, so your body Supplies more milk. If you choose to further reduce or completely eliminate(!) solids, this will greatly boost your supply within a day or so.
Make sure you are drinking enough water yourself. Your body will have a heck of a time producing milk if there's not much liquid in your body to make it!
If you feel the need to boost your supply, there are many ways to do it. Before you choose to use herbs, the two easiest (and cheapest) ways to boost your supply are:
*1-Breastfeed on demand
*2-Wear your baby. At 6mos he's probably not crawling yet. Sling him while you go about your day. The more you can be close to him, skin to skin even, the better for you supply - nice little hormone boost for your milk supply.
As for herbs, read up on it first as Amber suggested. Fenugreek and Blessed Thistle are the magic combo for most women when *1 and *2 don't work. When it comes to Fenugreek, you know you're taking enough when your body fluids start smelling like maple syrup :-) The two prescriptions that docs do the most are Domperidone and Reglan. From what I've heard Domperidone is helps more with fewer side effects. Just be aware.
Frequency: At 6mos, neither of my babies showed interest in food (just bowls, cups and spoons) - they nursed 10+ times a day with a couple night nursings as well. I intro'd a bottle of water to both at 6mos because it was summer and I wanted to be extra sure they didn't get dehydrated at all since I'm not always the best at drinking how much I'm supposed to :-)
You asked about cutting back on solids: I strongly recommend it! Again, if it were me, I'd completely stop until I was really really sure my baby were well hydrated.
Please, whatever else you may do, please do NOT intro formula at this point. There is simply no need. Your breast milk is the most perfect food for you baby. The American Academy of Pediatrics, the World Health Organization and the Institute of Health as well as every other Health/Medical organization out there all agree on this! Even the formula companies say this! Here's a link to an Enfamil article - they realize and understand that they are trying to emulate nature and can't quite get there!
http://pediatrics.about.com/cs/nutrition/a/enfamil_lipil.htm
All that to say, please don't spend your hard earned money on something that isn't as good for your baby as your milk is! not when you already have a nursing relationship and supply established!
That's all for now. I know this was super long, but I hope that you find it helpful! If you have any questions or if something I wrote sparks more questions for you, please don't hesitate to email me!
K. H, mama to
Catherine, 4yrs
Samuel, 14mos