Bit slow on this one but my two pennies is that your DSS will be well versed in falling in to line when he's at Mum's and behaving in an acceptable way for her else face the "rage" so he will probably be able to adapt to what is happening at yourhouse without slipping up at Mum's. I think it is wonderful for him to have the chance to experience a normal parenting model where people actually praise and encourage him when he succeeds rather than holding him back out of some weird narc hunger.
The only advice I would give is to make sure the things he does (at least at first) are for himself rather than for you as Mum (who almost certainly grills him on what happens at your house) will quickly twist things if she hears that you've asked him to do things that she perceives to be jobs for you. That definitely happened to us. We thought we were giving DSD life skills (because as any parent worth their salt knows, its a damn sight harder to get children to do things for themselves than to just do them yourself!!) but Mum had her beleive that we just wanted her to be our skivvy because we (meaning me - selfish hard nosed workaholic bitch excuse for a mother that I am) were too tired and more interested in work to do housework and care for the younger children. DSD ended up with a real cinderella complex and genuinely beleives that it was cruel of us to give her responsibilities.
I have the same situ as NADM. MY DSS is 12 though. It does take time and I wonder if all the good is undone down the road at mum's. However as he has matured I think he has adapted to the very different types of parenting.
It also helps that mum and I are very different people, too. She has been SAHM for 18 years and I work full time in a very stressful and emotionally demanding job (which often takes over my life) by way of example- he understands that I often don't have the time to "faff on" with things.
Go for it, from my experience kids in situations like this are adept at falling back into the behaviour they know is expected with the other parent. However, DSS seems to have grown in confidence, not necessarily with his mum, but in himself since DH and I took a conscious decision to stop babying him in the way he seemed to expect.
Personally I think you'll be doing him a favour. He's going to have to learn those skills at some point in his life and learning them now will be better than when he goes to uni/gets his own place. You'll be helping him to be more confident too which can only be a good thing.
I understand your concerns about his DM but many children learn about different rules at different houses and this is in the same category.
Regulars will know how frustrated I have been with DSS inability to think for himself - and lack if desire to become more independent.
DP has chatted to a few professionals who we've been involved with - family support worker, youth counsellor etc and they've encouraged him to develop DSS independence as much as we can while he's with us; give him tasks where he has to problem solve, think for himself, deal with the unexpected.
But, if DSS develops more independent thinking, he is likely to get into trouble with his Mum - she yells at him if he doesn't do things her way, or takes over if its taking him a bit longer to do something that she wants him to do. Her philosophy is if you need something done, do it yourself because otherwise its not done the way she wants it to be done - that extends to DSS personal care (she dresses him after swimming etc), and preparing school/handover bag etc.
Are we setting DSS up to fail if we begin to teach him skills that will help him in life but that he'll have to suppress while with his Mum?