I'd personally bring it up in the context of your baby.
As you will expect your DS to do basic tasks as he grows older you need to agree these household rules now - not in 5 years when step children have got used to a set of behaviours they are now expected to change.
Equally in the context of your DS, you can't have a set up where he (and any subsequent siblings) get treated one way and their half siblings another - unless your actively trying to create an environment that's trying to build resentment and have no desire to facilitate a positive relationship between all the children.
This has to also mean that household tasks are not put off on the EOW your SC's are with you - effectively leaving double on the alternative weekend for the rest of the household.
It sends a message that the household is totally run around the SC - which isn't healthy for anyone.
I can understand why he's saying what he is, but he's not thinking about the long term consequences.
Being a parent isn't always about doing what makes your children happiest and that can be very hard when you're not the resident parent.
Time with them is precious so arguably it's even more important when they are with you to set good expectations of behaviour and age appropriate responsibilities as opposed to providing the equivalent of a bi-monthly holiday environment.