I've noticed a pattern with Alex's toileting. Every two to three weeks, he has a small success on the toilet. It's usually preceded by a willingness to sit and try for a few days. After, he tends to go back to his refusals.
To me this suggests he does have some measure of control but something about the process makes him uncomfortable. It's not worth it most of the time but then he'll try it for a bit and once he's gotten his reward, he backs off.
Hopefully it means I can also back off my search for a "standing toilet" to install. (We had theorized that after almost twelve years of going while standing up, the transition to sitting might be too hard.)
We're cutting back on his laxative to see if that will help make it easier for his body to get rid of everything at once. Right now we get a lot of dribbling throughout the day, although he will remain clean for school or outings.
One of the big things I need to be clear on is that these are all baby-steps. There are no "breakthroughs" or "big moments". At this point, we can't even be entirely sure that the steps are in the right direction. We may be solving one problem only to put another two in its place (I've done that before with eating and dressing).