Yvonne ~
My son was 27 when I received my AN diagnosis, underwent surgery followed (as a plan) by FSR, so I can't offer any personal experience. However, I'm a parent and at one time, my son was seven years old - and I recall that time quite vividly. It was the year he asked me, with all the skeptical innocence of that tender age, if Santa Claus was real. I had not prepared for this (we were alone at the time) and had to improvise. I asked him what he thought (to find where he was on this issue). He was learning logic and he couldn't understand how Santa Claus could be all over the world delivering toys within a few hours. It just didn't make sense to him (but I could tell he was dismayed by his own skepticism). I asked him if he understood what a 'symbol' was (I used 'Uncle Sam' representing our country as an example). He did understand the concept (clever boy). So, I gently explained that Santa Claus was a symbol of the spirit of Christmas. He seemed kind of relieved. We're Christians, so I went on to (briefly) explain the correlation between the sacrifice of Jesus a (a gift) and the custom of exchanging gifts at Christmas. I added that Santa Claus is for the 'little kids' (remember, my son was seven years old at the time) that are too young to really understand the meaning of Christmas. It went well. Even my wife was satisfied with how I handled this parental minefield. My son enjoyed Christmas just as much as he ever did, even without the illusion of a magical man delivering his gifts.
I relate all this to make the point that, as the cliché goes: kids are resilient. You can be honest but keep it simple (your son is only 7) and don't offer too many details...he really doesn't care. He just wants to know that (a) you'll be O.K. and (b) that this won't happen to him. I wouldn't discuss how you'll be afterward, because you can't really know exactly how you'll be post-op and there is no need to burden a little boy with scary warnings about Mommy being 'different' after her operation. That will only frighten him. Kids need stability in their lives. If you have problems post-op, deal with those then. It's fine to be realistic but don't 'buy trouble' and don't put unnecessary ideas into a young boy's mind that Mommy won't be 'normal'. If that turns out to be the case, deal with it, then, as needed. I would also strongly suggest that you be very aware of your attitude/body language when you talk about this with your son. Kids are very perceptive and if you tear up, your voice cracks or you basically act depressed and a bit frightened, he'll very likely pick up on all that, no matter how soothing your words. Fake some confidence if you have to - for his sake. No need to lie and say it's 'nothing' but there is also no need to act as if you're going to the gallows on Surgery Day, at least when discussing this with your boy. I would be as 'matter-of-fact' as possible and tell him he has to 'help daddy' and that you'll probably be sleepy a lot when you return home from the hospital - and leave the post-op explanations at that.
Finally, the reality is that you know your son a lot better than anyone else does and that should be a guide for discussing this with him. The important thing to remember is that, at seven, a boy looks up to his mom and despite what may happen with the surgery or after, he'll still love you. That is what matters most - so don't ever lose sight of that fact. I wish you the best with your surgery and want you to know that you'll be in the thoughts and prayers of many as you approach this challenge.
Jim