As a mother, you could have said no to all of the treatments and you could have seen another doctor for second opinion. It wasn't a life threatening situation. If you really believed in your own judgement, why did you let your dear son go through the mess, especially when you know that he had bad reactions before.
I don't know why your son has gone through 3 operations before at such a young age, but based on your story I can't hlep but wonder if the previous operations were really necessary and if you thought long and hard before allowing them.
I agree that the doctors are terrible in this case. But I find it amazing that you as mother don't feel guilty at all for putting your son through all this. The bottom line is, when in doubt, you could have chosen the least intrusive / harmful treatment, which sometimes means simply wait and see.
I sincerely hope you can be more careful in the future when it comes to your son's medical decisions. Please think it over before agreeing to the doctors suggestions.
Hope your son can recover well and never has to go through anything like this again.