Who prescribes the medication for your son? I would suggest talking to him/her about your son's behavior and ask if it could be related to the medication. How come you switched from the Strattera if it was working okay? Also sometimes the combination of counseling and medication is most effective.

I copied this from the dexedrine prescribing info:
Aggression: Aggressive behavior or hostility is often observed in children and adolescents with ADHD, and has been reported in clinical trials and the postmarketing experience of some medications indicated for the treatment of ADHD. Although there is no systematic evidence that stimulants cause aggressive behavior or hostility, patients beginning treatment for ADHD should be monitored for the appearance of, or worsening of, aggressive behavior or hostility.

The prescribing info also says dexedrine is not recommended for children under age 6. You can read more at http://www.rxlist.com/dexedrine-drug.htm#