Seems the majority on this post haven’t had kids/divorce/remarried etc (yet), otherwise they’d be more forgiving.
Or has he never really grown up and just lives with his mum, just because. Well, because it’s easy.
Or maybe Mum had a 5h1t divorce and wanted him around, maybe Dad died – could be loads of reasons.
The OP starting a new phase in the relationship. Why have some gooseberry there just because they can’t be bothered to stand on there own two feet.
For how many years should the OP subsidise the lad? One imagines in this modern world of which you speak, the OP is already supporting another household.
One can be sympathetic without paying for it. I’d support my own offspring, and theirs, I do, but I’d draw the line at someone else’s. If Mrs Z ever tires of me, and most days I marvel that she hasn’t, then if I entered another relationship no children would be a prerequisite. Mind you, they do say if it’s got tits or wheels, rent it. Perhaps that’s the way forward.