Why can't he get medication for the entire day? At least for the time being?
I always like to compare it to a kitchen ;-)
Just imagine you've had a messy kitchen all your life. All of a sudden you manage to tidy up and get it all shiny and sparkling. The telephone rings and you walk out of the kitchen but it's the wrong number. Two minutes later you are back and you kitchen is in a right state, worse than ever before.

Dirty dishes everywhere, food everywhere, tomato sauce on the walls, flour distributed to remotest corner of your kitchen, puddles of lemonade, beer and milk on the worktops, table and floor, food rotting away on the table ...
Would you stay calm and cool?
That is what it is like when a rebound "hits" you. And all that once a day, every day.
While on medication your thoughts, your perception, almost everything inside you is in balance and all of a sudden everything goes haywire. You don't have any routines or any behavioural patterns to fall back on because you've never had the chance to learn these.
Of course, it is important to learn strategies to cope in such a way that they are automated. (That's the aim of behavioural therapy.)
A lot of our immediate reactions are automated over the years. You just react without thinking about it (like driving a car, after some time changing gears etc. is not a matter of "thinking about it", you just do it automatically). Once you manage to replace some unwanted immediate reactions and replace them with other automated behavioural patterns you may also be able to apply these when you don't have the aid of medication.
It's not easy, especially not at the age of 12 (it's easier with younger kids), and my experience with our kids is that this doesn't work as long as the medication isn't right. Our 13-year-old daughter is also very aggressive, not so much physically but with words, bullying her brother and sisters etc.
Even though she was on medication it was getting worse and worse. Recently we put her on different medication and this works a lot better. Now we are getting somewhere with her behavioural problems, it seems for the first time she actually understands what we are talking about. We are all working hard on it (always have done) but for the first time it seems more than another wasted effort.
There is always hope!