Between the school, tennis, and helping his tennis coach he has a pretty full schedule, especially for a kid with some extra considerations. I agree with the person that said making him responsible for things like his clothing, phone, recreation, etc. is a good first step especially if he has an account that money goes into regularly.
FYI - Given his diagnosis's your states Vocational Rehabilitation office may have a lot of support for employment and/or going to school. Hopefully he is making contact with the community colleges disability services office. His goals need a lot of planning and he'll need a lot of help with that planning.
If it were me I would not make him work and attend Community College right off the bat and insist he pay rent unless said rent is coming out of a support check. I don't know the details but usually not all of a support check is meant to be spending money or go straight to the kid. Generally it also covers necessities like food, shelter, clothing, etc.
Anyway - my point was going to be that even really good people don't always understand learning disabilities. As you know they are real and they often require accommodations, which means that maybe your son will not do things at the same pace or in the same way as the other kids in your new blended family. If your significant other doesn't really grasp that, to bad. You know your kid.
The basic tenants of responsibility, sharing in the household, accountability etc. are all great and real and absolutely necessary to learn but the journey may not look the same for your son. It kind of sounds like your significant other votes for tough love and that will result in a shift where your son steps it up to do things without lists and paying rent will somehow result in a kid that looks more focused and driven. Welllll.....it doesn't really work like that in my humble opinion. You know your son Liz. You know when he is truly being a lazy somewhat entitled teen and when he looks lazy to outsiders that don't understand. I know you can see the difference because you are smart and you know your son better than anyone - so don't second guess yourself.
My bossy advice - Put him in charge of his own laundry.
and quit thinking about his dad and his dad's house. You can't second guess all your decisions based on the fear that he'll go to his dad's house or all these different scenarios going through your head. Just do the next right thing.