Honestly? (And I'll probably get flamed for this) I'd probably frame it to the kids as "wow! Your dad doesn't think much of your capabilities does he? I know you're very grown up boys and more than capable of doing X y z yourself - just like your friends the same age"
Have they friends a similar age who do more? I'm sure they do!
"So eldest puts his foot down." So you put that foot back where it belongs!
"I suspect the 12 year old at least will get laughed at if he says he expects his mother to wait on him all the time" unfortunately these days I'm not so sure, from being on mn there's an increasing number of parents of both sexes who are mollycoddling their kids these days!
And similar to a pp I've been a mature student at uni and noticed a worrying number of students who had zero clue how to do basic household tasks!
Plus my own dd (18) has friends her age who aren't allowed to use a kettle or sharp knives! Fucking ridiculous!
Although I have to be honest op you sound quite passive and you and your current partner are being FAR too subservient to your ex.
Stop pandering to him and trying to soften up the kids. I very much doubt he would actually want them full time and I also very much doubt they would actually want to be there full time.
Kids aren't stupid they will play parents off against each other if they can, stop playing your exes game and run your household as you wish.
"When they are with him he just works and leaves them with his young gf who has said she "Doesn't really like children" so mostly stays away from them." Yea, no way he'd take them full time!
"If you don't let him grow up you are just causing him unnecessary problems" omg why on earth haven't you used this?!
I know it's hard but you need to find a way to stop cowering to him. And certainly don't allow your kids to abuse you as his proxies!