Not sure I can reassure you but I can tell you my own experience.
Everyone is different and some will have lots of side effects from medication (ANY medication) and some will have none at all. Most fall somewhere in between.
My son, now 16, takes both Ritalin and Strattera and we're just about to increase his dosage of Strattera. So he's medicated all day, every day. He doesn't have weekend breaks because he's home educated so it's not just for 'school'. He needs his medication to function, and that's all there is to it. Without it he is quite hyper and can't focus, so he's very disorganised with even the most routine of tasks.
Sometimes I think we need to separate ourselves from the negative publicity of ADHD medication. It's often seen as a bad thing to resort to drugs for this condition, but as a society we have no problem with medicating others, often when there are other options. If your son had an infection you wouldn't hesitate to give him antibiotics, but we wring our hands over methylphenidate and atomoxetine like they're the devil's work. They're not - they're just medicines to help our children cope.
You'll go through this same issue every time your son's medication needs to change - and it will, often, as he grows and the dosages no longer meet his needs. My son has been taking medication for five years and I've lost count of the times we've had to adjust - it's just part of the package. Ultimately though the only way you'll know if Strattera suits him is to give it a try and monitor it closely. Let school know about the change and ask them to report any differences so you can relay this back to the consultant. If Strattera doesn't suit him there will probably be other adjustments you can try.
As an aside, Medikinet didn't suit my son at all, and the only brand of methylphenidate that worked for him was Ritalin. They are meant to be all the same but they do affect people differently, so it might be worth considering a trial of different brands too.