Amanda you've taken on board the criticism and have used it to make changes as suggested, that is hugely positive and I'm sure that will go a long way to resolving the situation.
Some of the behaviour may just be stroppy teenager, you are right he should do simple things as described in your latest posts, no excuse for that level of laziness you are not a maid service and your dh needs to be on your side on this, your later posts do indicate that your dh and dss have 1:1 time.
You can't be all bad if he wants to come on holiday with you.
I think him feeling confident and at ease with his younger brother will take longer, don't force it but quietly encourage it.
Your dss has now seen that you have been prepared to accept you were wrong in the way you handled the room issue and are doing your best to put it right he should therefore realise that he now has to step up with the basic courtesy chores.
Can you talk to him without sounding as if you are nagging, I know it's hard to do so but you would reap the reward if you could, but honestly your dh does need to get involved here he should not be shying away from asking his son to do basic tasks as that is just showing respect and consideration.
Good luck, I think things will work out as you have shown you are the kind of person prepared to hear and act in order to make things better.