We are dealing with this right now. In my DS9's case, his upper jaw is too narrow and too far back. The palate expander was put in in November. We had to do 30 turns over 60 days and he just got the facemask yesterday. I was worried about his sleep last night but it didn't phase him at all. In our case, the ortho is pretty opposed to making kids wear the mask "in public" so we have to try and get 12 hours of wear in every day at HOME, which is pretty easy. He wears it right after dinner, takes off for his shower, and then back on all night. He has not complained at all this whole time. He has needed no pain meds at all. In fact, he looked forward to the expander turns every other day! He thought it was neat how it was moving his teeth. The facemask is not uncomfortable at all. I had the same dental issues as a child and the same treatment plan, but my appliances and mask were positively medieval looking. Things have come a long way in orthodontics and even the palate expander is a lot less bulky than what I had. We got a fancy Waterpick that my son loves to use and that helps a lot. So far, he has been a trooper, and I will tell you, this is a kid with autism and some major oral sensory issues and he is fine with this.
I will say, it is extremely important for you, the parent, to adopt a matter-of-fact attitude about this kind of stuff. Do not feed into your kid's drama by displaying your own anxiety. You have to be very mindful of the mirror effect. Of course, you can be worried about it all, but you can't let the child in on that. It has to be a "this is what needs to be done and that is that" kind of thing. Underbites, untreated, can end up causing MAJOR problems down the road. This is not a cosmetic issue like crooked teeth. A child had head/jaw bones that are not yet fused together and when done at this age, this treatment plan causes NO pain. At most, they feel some temporary pressure. As an adult, the correction is a very risky and painful jaw surgery with a very long recovery time. That is not ideal under any circumstances.
Get a second opinion, but this is the kind of issue that can really only be resolved with the treatment plan you were given. Explain to your son that this has to be done and it really sounds like a bigger deal than it is. FWIW, my son is the "cool" kid in his class because he has braces and no one else does.