My son wore a helmet for 10 months to correct his head shape, beginning when he was 7 - almost 8 - months old. We started with having it on all of the time and were told, as you probably were, that wearing while sleeping is most important because that is when they do most of their growing. He was used to wearing it for most of the day so napping with it and wearing it to sleep at night weren't that big of a deal. I had expected it to be a harder transition, but he's a pretty easy going kid and I think that worked in our favor. For us, it actually got harder when he was older and only wearing it at night because #1 - he got used to not wearing it and #2 - he figured out how to take it off.
I don't know how many hours per day your son needs his helmet. I started out with my son wearing his almost 24/7 so he could get used to it and that seemed to work. The not sleeping could be because the helmet is uncomfortable when he is lying down. The doctor, or whoever is overseeing this process, can take a look to see if it's adjusted properly. Sometimes after a size adjustment we were going back every few days to make sure it was 'just right.' The rule I was given was that if you take the helmet off and any mark remains that doesn't go away after 5 or 10 minutes, the helmet isn't fitting correctly.
The helmet is just one of those things that they have to get used to. And, in our case, we as parents had to get used to. Our son is now is now 20 months old, has been helmet free for almost two months, and has a beautifully shaped head. There is a light at the end of the tunnel and I know that light is hard to see when you are sleep deprived. I am so sorry that you are losing sleep in the adjustment process. I hope everyone adjusts and gets back to sleep soon. Good luck.