So .... you moved in and, because of your monthly contribution, your DP gets to keep the property he was otherwise going to have to sell because he wouldn't have been able to afford it on his own ?
For you to make these contributions, you have to work several part time jobs in addition to college, meaning you have little free time, and no money left over to treat yourself. Meanwhile, his life carries on as normal - like it was before you moved in. Nothing has changed for him .... except (I expect) he now has an extra pair of hands around the house (bet you do most cooking/cleaning?) and (sorry to be crude) the "convenience" of sex on tap. It's win-win for him and lose-lose for you.
I don't see that he's supporting you at all .... though I'd expect he'd argue that a couple of nights babysitting is supporting you .... when I assume that entails him sitting downstairs watching TV/surfing the net/eating sweets/listening to music - hardly a hardship when he already lives there !
Seems to be like the mutual support you'd hoped for is all one way. Your loss (when you moved in) is his gain, and to top it all, his son is effectively being supported by you too if he's making no contribution from his earnings. All this must surely affect your children too ?
Totally agree with NRIF .... sounds like you'd be far better off without him, and if that means he can't afford his place without your help then tough - his problem. Think it's absolutely disgusting that if you're meant to be in a mutually supportive relationship he gets £500 to spend as he pleases while you're half killing yourself to get enough to pay him his "dues". I can understand that you may want to work so as not to feel like a freeloader but a decent partner would really appreciate that effort and insist that once all essentials had been paid for, you go 50:50 on what's left. A decent partner would NOT also disappear all weekend while you're stuck at home with the kids. Yes - know they're not his, but he's not treating you with any consideration at all - nothing like an equal partner certainly .... I really do think your only use to him is as the means with which his life as a single man carries on as per usual and as the solution to him otherwise having to downsize. You deserve better than that and so do your children.