I agree youre doing the right thing, I'd lay odds even if he goes to his dad's, while dads wife may be a mug, I can't see Dad tolerating being talked to like shit on his shoe, paid no keep and disrespected so wouldn't be surprised if he's begging to come back soon!
My experiences are these:
At 22 I returned home to save up for my wedding. I paid 1/3 of my take home for my keep
(I don't understand parents that don't expect working children to contribute financially, they cost in terms of food [and boys especially can eat a lot!], electric, gas, water [especially in England], WiFi, council tax, phone bills).
I worked full time as an HCP and so shifts but still cooked, cleaned (dishes, worktops, dusting, hoovering), did laundry, food shopping, waiting in for deliveries etc, feeding and walking the dog.
When I went back to uni as a mature student I mixed with students of your sons ages, they'd mostly return home during the longer holidays, to see family, but also for holiday jobs and to save money. I can't think of one of them that would ever disrespect their parents like that! Not all of them paid keep but certainly the ones with less wealthy parents did. They certainly were expected to help with housework, cook at least once a week (or pay for takeaway, some were awful cooks!), and if there were very much younger siblings babysit/help with holiday childcare (unpaid). None of them begrudged this!
My own dd just turned 15 has helped since she was little (to be fair it's a novelty 'helping mummy' when they're litle eh?) But now she keeps her own room spotless (she hates mess or dirt), does her own laundry (that's come about over time due to I'm allergic to her detergent and vice versa its a pita! Also as a teen she's quite 'I wanna wear x y z at the weekend' and it's quicker and easier for her to throw what she wants in rather than her explain to me which pair of identical jeans she wants washed!), does the dishes (we do this sort of 50/50 depending who's doing what that eve), cooks at least once a week (she proudly tells people pasta arrabiata is her speciality it is rather good!) Runs Hoover round if I'm having a bad day (mobility and pain issues), helps with the grocery shop. We have a chalkboard (my meds affect my memory) and when we're running low on loo roll, milk or whatever, whoever notices puts on chalkboard for me to put on shopping list and it's not always me that does that. She comes up with ideas to make things easier too. I'm lucky to have her.
But yes I have a disability and mh issues and so people think we'll of course she helps.
But her little group of 5 that all go about together, are all the same, they're all 15/16.
2 other families like us are LP. Both those girls help as much as dd does and one has a baby sibling who she babysits, baths, changes nappies etc.
The boy in the group also helps just as much, no way would either of his parents brook any 'that's womens work' crap! They've 2 of each and all the kids help out, both parents work full time in physically demanding jobs.
The last girl is the eldest of 6 and parents work full time (dad actually works 2 jobs), they're all expected to help even the youngest who lays and clears the dinner table and feeds the cats. The eldest, (dd's friend) babysits regularly and helps the 2 youngest get their stuff ready for school each night as well as helping out with housework as much as my dd helps here if not more.