Thank you so much for all the replies and understanding! It helps so much to know its not just me.
Normally when we have him for a week, my dp's mum (who is retired) looks after him for some days, I look after him for one or two days, and my dp tries to book some annual leave.
My dp is very clear that I don't have to look after him and would never expect me to, so that's good.
However, my dp's mum has hinted massively recently that she would like me to look after dss more often - my dp actually had to have a chat with his mum about it as he didn't think it was ok for her to put that pressure on me, which is nice.
So I don't know how its going to work, but I know my dp won't let the responsibility fall on me.
So, there are some days were I have time to myself for part of the day. So maybe I shouldn't complain. But its not the same as knowing I can just relax, I feel like I spend the whole day knowing that its going to be really noisy and busy later on. And I get no time just me and dp, none at all.
The thing is, when its just me and dss he's like a different child. Obviously I have to discipline him then, which I'm sure my dp must realise. His behaviour when its just me and him is fantastic. I think this might be because he doesn't have that 'safe' feeling that he would with his dad. Like, with his dad, he knows he can push it but his dad will always still love him, he's his dad! But with me, I think he doesn't have that same feeling if that makes sense. Like sometimes the closer you are with someone the more you push it.
But, I think also, I am quite firm and strict. There are boundaries in place when its just me and him.
Also, some members of dp's family, and dp occasionally, treat him like he is a baby and let him get away with anything. Dp's mum says she does this because she feels sorry for him with his parents being separated. I think its doing him more damage though.
Dss will tell them to put his shoes/coat on, get him a drink etc, and they will! He is perfectly capable of doing these things himself and I never do them for him, I just playfully say, oo, you can do that yourself!
I think treating him as though he is alot younger makes him act younger and encourages him to have tantrums.
But I've gone of topic there, its just something I've noticed!
Just to say though, my dp is a wonderful dad and partner, but we do need to talk about this. I'm worried that he'll take it to mean I don't care about his son, but if he's going to be here for five weeks I think there should be house rules and boundaries set, and I should be involved in that.
I sometimes feel like I'm only involved in the 'fun' stuff, and feel guilty if I don't take part enthusiastically, but anything else, like rules and boundaries just isn't my place.