I am in a similar situation, but it would have been impossible to move closer to my partners ex as she moved over 100 miles away a few years back and our families, jobs and lives are where we are now so the option of moving was out of the question.
I also have a 6wo daughter with my partner who also has a larger room in our house than his DsD (she’s not in it yet will be at 6 months) she has the larger room due to our daughter being with us 100% of the time and OMG how much stuff do baby’s come with!!??
I agree whole heartily that you’re right in saying your son should have the larger room as he will be with you the majority of the time, and it’s just the principle of it you’re paying for that house so you should get the say!
It does sound like you’re having to compromise an awful lot to help your DP out. When it does come to the mortgage and buying a house have you been to see a mortgage adviser? My sister is on a fixed term contract with her job and she struggled with a standard mortgage however her mortgage adviser was able to find something that suited her. It may be worth a look? That way if it possible it does not fall on your shoulders?
I know many dads would fall over themselves and go to the ends of the earth for their kids, however sometimes they do this without thinking about the implications it has on their current partner. For example I asked my partner to take the day off work in order to come with me to LO’s 8week appointment, I will admit I’m a massive wimp and I’m dreading seeing the LO in pain after she gets stuck with the needles :-(. However his ex has asked him to have his daughter an extra week which he has agreed to, however it now means that he can’t go to little ones doctors appointment, as he will have used up all his holidays. Although I don’t mind because it’s lovely having DSD here longer due to them living so far away it does mean that I’m doing the lions share of the work for our daughter including doctors appointments. I also asked my mum to come with me to the appointment so it’s not a problem.
Also bare in mind if you move away from your support network it will be a lot harder for them to help with your little one.
I think you should write down a list of what you’re worried about and give it to your OH, that way you can talk it through and see if you can come to a solution?