Hi Tom
I am a step mother to 2 boys 22 and 15 and I really step back and let their dad be the parent and I am just an older friend to them.
I always bite my tongue and let my husband make the decisions about them. I only get involved if he (or the boys) ask me. They come to me way more than their mum as i have approached things this way.
Dont get me wrong - there are times when its very hard but it works for us.
Your son is YOUR son, the house is YOUR house and YOU should make the decisions.
If you change things because of your gf, you son will resent her and at the end othe day its his home.
I would say also that 90% of our arguments are over the children and exes - I have 2 daughters 16 & 18.
However, your not doing your son any favours letting him do nothing. He will cope better when he leaves home if he knows the basics. He should really do his ironing and put dirty plates etc away.
Good luck - I know its hard!!