Hi C.-
My son had his tonsils and adenoids removed in May and the first week was really hard. He pretty much stayed in bed all day and was either asleep or if he was awake he was complaining about it hurting. The only thing you can do is make sure he's taking his medicine, drinking fluids and eating a lot of popsicles.
Still to this day, when my son doesn't want to do something he will use the excuse that his throat hurts to try to get out of it. I attribute this to the amount of babying I did to him right after the surgery. I don't regret doing it since I know that he was in a lot of pain, but just be aware that because of the added attention he's getting (especially with a younger sibling pulling you away from him) he may hold on to this a little longer than he needs to just to continue the attention from you.
As far as suggestions go, my son liked to take baths to make him feel better. I also bought him some new colored pencils and a lap desk so that he could draw while in bed.
Once you have your follow up visit with the doctor and they confirm that he is healing just fine, if your son still seems to be "moping around" maybe plan a little something special to get him out of the house and tell him that when he thinks he's feeling better you will take him to ________. This should motivate him to feel better soon!
Good luck,
K.