Personally I would never risk a such a secure financial position but especially not with a five year old in the mix.
Your P paying half bills actually won’t make up for the shortfall. I was left so much worse off when my ex moved in for a few years, and I wasn’t in receipt of any benefits. I lost my single person discount on council tax, bills went up, he was reluctant to split them equally because I had a child and would complain about every expense until in the end I just paid everything, he would buy expensive non-essential groceries and expect me to pay half…. I mean, he was an all round dickhead for many reasons, but there was a lot of stuff I couldn’t have predicted beforehand - some of which was discussed in advance but the reality didn’t match.
If he moves in is your P prepared to view the three of you as a unit financially? (I am only excluding your 18yo from this as he is contributing - but it’s worth recognizing that you may well want to treat the eldest, and/or pay for things for all of you, and your P might feel that’s not on him) Will your P want to split bills etc unevenly as you have two children in the house? (I’m not saying this is outrageous of him necessarily, just pointing out that you might not be reducing your outgoings nearly as much as you’ll be reducing your income). Are you comfortable working additional hours in future to make up the shortfall if that impacts your youngest and the amount of time and flexibility you currently have to deal with parenting him? There is a huge toll on MH to have to work long low paid hours when you’re also solely responsible for a small child. How much parenting do you want him to do (if any), and how much does he think he will be doing (pick ups/drop offs, sick days, school holidays etc etc).
Far more importantly long term, have you made plans to protect the house (I’m assuming this is your only/main financial asset)? Have you had legal advice, had paperwork drawn up for him to consider confirming he will never have a claim on the property. As a single parent, frankly, I think it’s lunacy to consider making yourself this vulnerable when the desparotu between your financial security is so vast.
If he really cared about building a life with you he would be willing to relocate closer to your home so that you can continue to see each other, while maintaining your financial independence. If he isn’t then it doesn’t appear that he is bothered enough about you to be worth taking this kind of risk for. Sorry.