D.B.
Quite a quandary - and I sympathize. We went through something similar with our son in high school. My mother had given him her car (which we were fine with), but after a phase of mouthing off, we informed him that someone so immature would lose access to the car. (Cue the "It's MY car!" and "But I pay insurance and maintenance and gas" conversation.) So, he was told he couldn't drive the next day, and to get up to take the bus (which was apparently the least cool thing he'd ever want to do). Alternatively, he could hitch a ride with neighborhood girls a year older - but he was too shy to ask them. We slept late on purpose so he had to get up, get breakfast, get to the bus stop (next door). Instead, he cajoled a neighbor to drive him! Resourceful, yes, except that he lied saying that we had already left the house. So, he lost the car for another week for lying. It took 2 days for him to be more contrite and turn around his disrespectful tone.
I don't know how much you feel his ADHD requires additional support from you, rather than just taking his lumps and getting all natural consequences. So maybe you still need to give him his meds, and maybe you still need to oversee his use of his funds. I think the ethics of using an Uber if they aren't supposed to is a problem, and I think the safety issue is worth discussing - what is the downside of being a kid in an Uber alone? I suppose you could say that, if he's too young to understand the risks, then you have to step in until he's old enough.
Part of me says, let him use up his money and then deal with the absence of stuff he really wants. But if he has substantial savings, it could take a while for his account to run dry. I also don't know if you would be undermined at all by your ex or the grandparents. If you have exclusive control though, you could move some of his money into another account, saying if he's not responsible enough to go to school to get an education on how to survive in the world, he's too immature to have access to $500 (or whatever it is). You could give him, say, $100 for discretionary income - but that's 10 rides to school and then he's done. Just about the time it's getting cold, you know? I suppose you could go to the parents of the kids with the accounts and say you don't want them to use their accounts for your kid or at least inform them that their kids are lending the accounts to their friends (or at least your son, who paid back once but won't be able to sustain that), but that's labor intensive and doesn't do as much to increase his responsibility sense as much as you might like. It also presumes that you know all the kids and their parents, and that getting the adults involved will work in your favor. I also think, at some point, his friends are going to get sick of bailing him out all the time too, which could either make him shape up or leave him friendless.
Breakfast: A friend has an early departure with 2 kids, one of whom (8th grade) has ADHD & autism. He needs help with the meds and so on. She does a lot of grab-and-go breakfasts, like egg casseroles (with green chilis, or with spinach, or whatever the kids like) that she cuts up like brownies and freezes in single serving packs. Your son could grab something like that to eat on the bus. Perhaps the kids could work with you on Sundays to cook up a week of lunches and breakfasts.
Running out the door with no shoes works on a mild early fall day. It doesn't work in the rain, snow, ice and slush. So perhaps he needs natural consequences there as well. Some of this is his choice to be disorganized, some of it is his ADHD. It's hard to straddle that line.
I went to a teacher/paren workshop many years ago, at which the leader taught the phrase "How unfortunate for you." Kids who got warnings of any sort (wear your gloves to recess, pick up your toys or they'll go to a toy-less kid, get up for school...) and then ignored them and started the wailing of "It's not FAIR!" pity party were met with "How unfortunate for you. I wore my gloves (remembered my lunch, organized my backpack last night...whatever the issue is) and I'm not having any of those problems." It does reinforce that the child's misery is the child's choice. It's one thing when it's a 3rd grader on the playground, unable to use the swings due to no gloves. But you're teaching life skills to someone who's going to be his own in a few years, so the stakes are higher.
Good luck and let us know how this works out!