THANK YOU SO MUCH. We are not alone! I am having a rough morning with trying to figure out what else to try. Thank you!
It sounds like your son has some anxiety surrounding medical issues and death. My son does too-- and we take him to couseling for these issues, so that he can express them and discuss them, and so that we can learn what to do to help him.
As far as the getting in one thing and can't get it out-- that's something that I have read is relatively typical of ADHD children. They "hyperfocus" on something and it just stays with them. My son gets this way about his games-- he will pick one game and want to play it ALL the time, then move on to another one.
Overall, if his medication is working well, which it sounds like it is, then you might just want to investigate some supplemental counseling for him to deal with anxiety and/or any other feelings he might be having.