robd I realise that it's all still very raw and new for you, however, there is a certain element of "waking up and smelling the coffee" that you need to do.
The financial aspects of your situation will cripple you and mean that you will be relegated to a 2nd class parent if you don't have the solid foundations of suitable accommodation for your son to stay o/n's at your gaff. How can you be taken seriously as a fully-fledged parent if you are living in a shared house?
I can't remember all the details of your original post, if there is property involved, but if you and the ex weren't married and the relationship hasn't been a hugely long time then why the hell are you settling for less than a 50:50 split of any equity???
It is unrealistic once a couple separate for the two parents and any children involved to have a lifestyle like one prior to the relationship breaking up. Both parents need to live within their means and for a wholesome environment for the child(ren) to group up in.
Why are you proposing £500 per month for child maintenance? The CSA guidelines are there and you should be proposing that - the rest of the money can be put towards something much more relevant to reinforcing and promoting your child's relationship and bond with their parent - and that is a home. Other posters have said that you should be able to rent a 1 bedroom flat and use a sofa-bed when your son has overnight staying contact at yours.
As a teacher you of all people should be aware of the benefit to children of an actively involved father in their lives - these include higher educational outcomes, lower rates of delinquency, crime etc (all confirmed by DfES, DCFS predecessor).
Isn't it worthwhile doing as much as you can to promote your child's secure relationship with his dad?
I can relate to all of this because when me and my ex separated I was in shared accommodation.
It wasn't great and I didn't enjoy it one bit - and the biggest point is that neither did my daughter.
The ex remained in the former matrimonial home (we eventually sold it and shared the equity 60:40 - which was used to pay off debts ... and financed the eventual family court solicitor leeches), my ex was pushing and pushing for daughter to stay o/n with me.... she couldn't, even though my daughter wanted to and was pleading that she could sleep on the sofa.... that was grim, really really emotionally tough. I did take my daughter to my parents regularly every month for a long weekend, but that was because I DIDN'T HAVE ACCOMMODATION OF MY OWN.
You have the chance to prevent this now and I highly recommend that you take the advice on this forum and especially that gillybean has offered...