Hi K.:
My son also had his adenoids out at about the same age.
He did really, really well. I expected him to act differently and to complain about some pain, but, he never did.
Since your sone (and mine) is a picky eater, how about just using this time to spoil him a little and use the excuse for his surgery??? I gave my son applesause, yogurt, ice-cream, sherbet, pudding... Fun things. Ice cream, yogurt and pudding have plenty of calcium. Organic applesauce is so good (you can get the no-sugar added).
He won't be little much longer. Use this spoiling, cuddling time to just enjoy!!
Good luck and have fun!