I see your predicament, he's not yours, so you can't do anything negative, like rant/threaten etc, since he'll just blank you out.
As I see it, you can be positive with him, hand him a job on a plate, basically, "go here, at "x" time on "y" day, speak to bob", which I think he will respond positively to.
Alternatively, you can be negative with his mother, and "suggest" she threatens him with rent/eviction, stops feeding him, stops doing any of his washing, and explain to her that while he does not need to get a job, he won't, it's basic human instinct to rest ready to fight for food etc, if he doesn't have to get his own food, then his instincts are just to rest, since he doesn't actually have to do...anything.
Your other option is one you've stated, and just stay out of it completely. At least this will save you from the train wreck that is his life. You could go one step further and act like he's not there, and randomly move stuff around in his room when he's out, goto the toilet when he's having a shower, generally anything that will bug him or make him uncomfortable, he'll either want out of his own accord, or he'll confront you, to which you can reply "if you don't like it, you're welcome to leave".
Remember, when arguing, that you and him are on the same level with regards to ownership of the house, his mum owns it, neither of you do, so you can turn this to your advantage because your mrs wants you there, where he's in the way (rephrase depending on how harsh you wanna be).
You should really have a word with your mrs, it's your "home", and it would be fine if he was a teenager, but he's an adult, and it's just a little wrong. He'll never get anywhere in life while living with his parents (girls won't be impressed etc), jobs above the lowest rung will remain closed to him etc. This is all ammunition for you to use in case you do get involved
Action needs to be taken now!