Seeking expert counseling is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
I believe that I can help.
It is wonderful that your son likes Seroquel, which means, in my estimation, that it is working for him. Seroquel is meant to be taken over the long term and is likely to continue to work for him.
Seroquel, like many under other antipsychotic and other psychotropic medications, is not easily discontinued in many instances, without difficult withdrawal symptoms. The withdrawal effects can be quite difficult for some and for this reason, your son's doctors will probably keep him on it as long as it is working, which may very well be for years. If he is stabilized they will not change.
If for some reason he had to come off of it (which is not likely) and they try other medications and they are not as effective, they might decide to return him to Seroquel if he is not allergic to it (and it seems that he is not).
There is a good chance that it will work again.
Your son's situation is looking up now. Keep your thoughts positive and try not to focus on the "what ifs", which will not serve you in any positive way, but rather feed your worry.
He will do well if he avoids alcoholic beverages, diet sodas, and MSG, and he should be encouraged to eat a healthy diet including much fruits, vegetables, lean protein and water. His diet will have a direct impact on his moods and feelings.
I shall continue to keep your son and family in my prayers.