Assuming this child has an aide, you could try some of these activities. If not, perhaps you can get some volunteers to help the child with some of these activities when they have a few spare minutes. When I had a child like this in my class, he mostly sat and listened and sang songs with us and did lessons with us, unless I had an extra volunteer that could work with him. When it was his turn to be called on, I would either change the question to an easier one, or have him try to repeat back the answer after me. When we did group work, I had alternate worksheets and alternate activities that he could do independently when possible while I worked with the rest of the children. Sometimes I just traced the correct answers on the worksheet everyone was doing, and had him try to go over it with a crayon. I also pulled him out of playtime and tried to work with him then. I accepted all offers of help, and whenever somebody new came into my classroom, I immediately would pull out a tub with activities and instructions in it that I had ready, and hand it to them. It also had a documentation sheet that asked volunteers to sign in when they helped, write the date, and what they worked on, which I have included here.
You might try doing some classification types of activities, like putting pictures of clothes together, toys together, etc. A simple puzzle, like putting the number one piece into the number one cut-out might work. If you have any pegboards, it might be good practice just to put the pegs into the slots. I have some peg boards that have numbers on them; the kids are supposed to put three pegs into the one with number three, etc. I had a child once that was working on just putting chips into a slot that I had cut out of the lid of a margarine tub. That particular child was better off counting things and putting them into the slot, because that meant that he could not stop to play with them after he counted them. It didn't stop him from playing with the chips before he counted them, though!
Another good thing to do is to find some plastic letters and pull out the ones for that child’s name. Write his name on a large piece of tag board, and have him match the letters to form his name. Matching any other letters or numbers together is always a good thing too, since it develops visual perception.
Sorting activities might be too hard for a developmentally disabled kid, but most kids can sort by color. In any case, he can practice identifying the colors at least. Another thing that the child might be able to work on is tracing his name with colored markers. My kiddo that did that a few years back needed his name printed very large- it took up the whole sheet of paper. I also drew some curvy and straight lines on a piece of paper with a thick black marker. I xeroxed the paper several times before I gave it to him, and I had him try it every day for a while. Again, he too the marker and tried to trace the lines. He was way off, but got better as the year progressed. You could also have such a child try to cut on these lines. Unfortunately, he began to hate the tub with his name sheets and tracing sheets, and would start to cry as soon as he saw it, poor thing!
My developmentally disabled student basically stayed with my class and did everything that the others did, but he scribbled on everything. The kids got used to it eventually, but I had to explain the situation to them at a time when he was not around, and I tried to persuade them not to tattle when he was not completing assignments correctly. They also tried to avoid him by refusing to sit by him, etc. That made me angry, but I had to again explain to them that what he had was not contagious, and that their behavior was hurtful.
If you can get your hands on some baby board books with just vocabulary pictures in them, then the child can try to read and name the pictures. For example, the book might be called "Farm Friends" and have just a picture of a goat with the word "Goat" underneath it, etc. You might also try to have the child string some fat beads on a thick string. I have some shoe laces tied to some thick pieces of cardboard. The cardboard pieces have numbers and dots on them. So the piece with a number 5 on it has five sticker dots. The children are supposed to lace 5 beads on the shoe lace, etc.
Another thing that is good to do is to draw numbers on paper plates with a thick marker, and have the child put that many objects on the plate. Now with a VERY low child, I would see if he could put just ONE thing on each plate with a number one on it. If he can do that, then maybe two things on a plate with number two on them. I usually scatter the plates around the table and have them put the correct amount of blocks on each one. They may have to put two blocks on a plate 5-10 times.
These activities worked because he was a very compliant child who didn’t really know that he was “different” from the others. Working with a less compliant child is a much different story, and I am trying to get that figured out this year! If I ever get it “down,” I’ll be sure to make a post on how I did it!