First, I am sorry that your family has had such a difficult time with this. It should go without saying that parenting in this kind of situation is exhausting, frustrating, and discouraging.
Second, where I don't know your child or the details, I won't presume to assume that I know what is going on, but I will say that, no matter what the cause of the tantrums is, it is critically important that you and your spouse come to some kind of consensus on how you will deal with them, and then do so, consistently. It may be that they are primarily neurophysiological in origin, or it may be that they are primarily behavioral in origin (most of the time, these things are a little bit of both). Either way, inconsistency across caregivers will only escalate the behaviors.
Third, whether or not he is making choices about the tantrums, or truly cannot control them (and I might closely consider the question of choice, given the abrupt cessation of his tantrums), it is also important that all of the adults maintain a calm, rational approach to dealing with him. (Believe me, I know that this can be very difficult, in the moment; sometimes one needs to step away for a moment, regroup, and then return to the situation.)
I agree that the first consideration is his safety and that of others around him. This includes the emotional trauma these kinds of episodes between him and his parents leaves with his sister.
A few more thoughts: I do know that, for some children, an approach similar to what you describe (exacting an actual monetary cost for damage that he does during a tantrum) has been effective, especially when it is primarily behavioral. You are also reporting that he behaves noticeably better when on his medication. Perhaps exploring medication or dosage adjustments with his medical team would be appropriate.
The work of Ross Greene is worth a look: "The Explosive Child",
www.livesinthebalance.org
