FIrst, I'm sorry to hear you are having such a rough time w/ the little one. I admire that you are doing it at all! It's a very tough job. Do you have anyone who can help you get rest during the day?
Second, have you heard of the book, The Out of Sync Child has Fun? I haven't read it myself but have seen it recommended for kids w/ sensory issues. Maybe there will be ideas in it for you.
Third, in the shower this morning, I was wondering if maybe getting him some or as much as possibly, sensory-kind of activities, may make him able to sleep better? Like, give him a bath (if he's okay w/ water) & let him splash, pour, mix, etc. until he doesn't want any more or you can't add any more water to make it warm ;-). Let him play in the kitchen sink. Put out a big plastic cloth (like a shower curtain size) if you think he'll go to town & let him play w/ rice on a cookie tray. Or shaving cream. Get a big bin & put sand it in. Get some plastic toys & let him bury them & find them. Shovels. Buckets. sifters. Etc. Just stuff to let him senes things & play.
Fourth, my ds1, who is 7.5, was recently eating us out of our budget. Knowing what I do of digestive health, I thought maybe he needed digestive enzymes to help him actually digest what he was eating, not just keep eating. If you think the little one will swallow pills, maybe try that? If not, you could probably put it in his bottle but it will start to digest the food when it hits it so you can't put it on/in something he's not going to eat right away.
Fifth, on eliminating foods - if he has issues w/ food (&it's pretty common, IMO, especially w/ children who have lots of issues that center in the gut, sensory being one of them), he may stop eating a ton once the foods he can't tolerate are elmininated. There's a yahoo group called foodlab that is very helpful for parents of kids who can't tolerate the usual stuff. THey're very pro-bf'ing but if you say he's a foster/adopted child, they'll be sympathetic, I believe. Also, on the gut stuff, you may look into the GAPS protocol which is designed to heal the gut. Many of the parents I've read about using it have used it for their ASD children. Since sensory stuff often goes along w/ ASD conditions, I thought I'd mention it. It is not easy, but I have seen it work. There's a yahoo group called GAPShelp about it too.
Best wishes. This little boy is very lucky to have you in his life. I truly hope that you can figure out a way to work w/ him so that you both get what you need.