You need to look up the Russell Barkley lecture on youtube, it's called something like 30 essential ideas parents need to know about ADHD.
He will be shattered/exhausted from work. He is working harder than somebody without ADHD would have to work to keep going. If he is on medication, it's usually timed to be wearing off just at the end of the working day (this is something that really gets my goat, but that's another topic.) If he is not on medication, then he's working doubly as hard just to meet a baseline that every employee will be expected to meet.
On top of this his executive functioning (essentially, "adulting" - any kind of life organisation, looking after yourself) skills are likely to be a few years behind what his actual age is, which is why he probably comes across as immature in some ways. So yes, while it is reasonable to expect a 17 year old to do their own cooking and laundry, how much of that would you expect of a 13/14 year old? Taking some of the load off him won't infantilise him if he just needs a bit more time to mature.
I can't do ANYTHING in the early evenings if I've been tired out by the day - I'm literally a zombie. I have burnt so many pans dry (alerted by the smell of smoke as the pasta/rice starts to smoulder) because I have absolutely no sense of time and it doesn't feel like it's been 10 minutes but actually it's been 50. DH bought me a rice cooker, brilliant thing because you can't overcook the rice. However if I was just cooking for myself and not the DC, I wouldn't even bother with this because I'd have to cook something to go with it (unless I'd done a chilli/curry/something else in the slow cooker earlier, perhaps). If I set an alarm then I turn it off and think it's been 2 minutes but it's actually been 20.
Could you have a bit of a brainstorm all of the adults in the house including the GF if you like - see if you can set up something like a laundry rota with him only having to think about laundry at weekends, a batch cooking rota so that there are meals to heat up in the microwave throughout the week, and he can practice some more basic cooking as and when he has the energy and headspace to do it. I rely a lot on convenience foods (and DH).
For the room maybe offer to help him get it organised if it needs it so that everything is easy to put away, but try not to control too much - it doesn't matter really if he's keeping clean clothes in a laundry basket and wearing them straight out of that, bypassing a drawer. (If it uses up a laundry basket that you need, buy another one or get him to.) It might matter if he's balling up wet towels and leaving them on the floor - perhaps get a hook or a rail for the bathroom. It might matter if he's keeping half eaten food in the room to go mouldy, so maybe do a call out in the evening for dirty plates (etc) to go into the dishwasher/washing up load, but any mess that is "clean" - that's really up to him to decide if it bothers him or not.
Help him identify at which points of the day/week he has more capacity and which times he just needs to chill with no expectations placed on him.