K.,
It sounds like a milk protein (casein) allergy to me. Both of my daughters had this. Some babies grow out of it, but the key is to not introduce casein back into their diets until 3 yrs old if you want better chances of them outgrowing it.
I breastfeed my daughter and once I eliminated casein from my own diet, it took about 6 weeks for me to see a complete change. I saw a little after 24 hours and so forth, but it depends on the person in question. I was surprised that it also made ME feel much better and I never realized I had a problem with milk or dairy products!
Once the proteins are completely out of his system, you might see an improvement in his sleeping too. 6 months is pretty typical age for a growth spurt and sleeping changes anyway, and then they wax and wane from teething, etc as you know. And some babies are frankly just lousy sleepers. It's nothing you're doing "wrong," it might just be the way he is. I have one great sleeper and one who didn't sleep through the night until she was 4 years old!!
Also, you might check out HomeFirst, they are a homeopathic medical center in our area that does MSA testing, which is a non-invasive sort of "accupressure" allergy and sensitivity testing. They put a wand on your thumb (or for babies on their back w/diaper elastic holding it in place) and it only takes about half an hour to get a printout. If you see lots of allergies they can suggest supplements to help with digestion, respiration, etc. too.
Remember, casein is hidden in lots of things- bread, crackers, etc. so make sure you look for it in all those places. It's in everything with dairy (whey, cheese, butter, etc) but some people who have problems with COW dairy can sometimes eat GOAT dairy just fine (those proteins are smaller, more like human milk proteins.)
Also, be aware that SOME (not all)babies who are sensitive to dairy will also be sensitive or intolerant of soy as well. You don't say if you have tried to introduce solids, but I would also delay that as long as possible. They longer and more slowly you can delay and introduce foods, the better chances are that he won't be allergic to them as well. My youngest daughter was not eating solids until 13 months old, and while this might surprise some people (and some said she would NEVER eat) she is now a great eater.
Hope this helps. If you have questions, feel free to contact me. You might also think about re-lactating for him.
-Amanda