Affective instability is essentially an unstable mood, mood swings, etc., which suggests a mood disorder of some type. At one end of the mood disorder spectrum you have depression, and at the other end you have anxiety. Some people suffer from one or the other, or a degree of both. It sounds like he tends to be prone to anxiety, which could easily explain the reason for his aggression in the past. Stimulant medications can exacerbate or worsen anxiety even while they are helping with focus and attention. It doesn't mean you should necessarily stop the stimulant, especially if it's helping his ADHD symptoms. However, it could mean that he needs an additional medication support to address the anxiety part.
Unfortunately, there is no black and white solution and this really is going to be a trial and error process. Don't be too hard on yourself or him or even the doctor because this all takes time and a lot of patience and careful observation. The good news is that there are so very many drug options out there that your chances of eventually hitting the right combo or cocktail of medications are pretty good. I highly recommend seeking out the help of a trained pediatric psychiatrist to help you manage these medications. A difficult child whose situation is more complex than a cut and dry diagnosis is usually beyond the training and experience that a typical pediatrician has and you will likely drag out the process longer than you would with a qualified psychiatrist.
