As part of getting Alex to follow directions without complaining, we're using a clicker. Each time he follows a direction without protest, he gets a click. When he gets 10 clicks, he earns a privilege (usually 20 minutes of screentime).
The downside of this system is that Alex is obsessively tracking how many clicks he has earned and will begin demanding to "earn it" repeatedly and loudly. On the plus side, it means he is sometimes independently looking for things to do, such as tidying up toys, in order to earn his rewards.
Nathan is also very interested in Alex's clicker and will ask to see it throughout the day. He will tell Alex to do something (usually non-sequitur like "sit on the chair") and then demand that Alex earn a click for it. We're trying to discourage him from giving his brother commands but not discourage him from feeling like a part of the process. It's a tricky balance and one I'm not doing great at figuring out.
The clicker is proving to be a very versatile tool though. Because Alex can earn a click anywhere we go, it means we're no longer tied to good behaviour in the house and then in a new world once we go out. He even does well at understanding that he will have to wait until we get home before he will get his earned prize.
I'm really hopeful that this plan will give us the tools we need to tackle toileting and other issues.