RUN away from this man. He is clearly giving you signals about what to expect in the future.
And if he's this argumentative and exploitative now (not recognising his use of electricity, petrol, etc, your financial hit through loss of benefits, previous old debts)... what is it going to be like when you are in deeper with this man?
As someone else said, talk is cheap. Yet he isn't even talking as though he has any idea of what "sharing" a household's work/money is like. He sounds like one of those guys who puts in X amount into a pot and expects you to foot the bill for your share, plus all the extras he argues he shouldn't pay for. I've got a friend like this, her partner (father of her kids) earns the same as her (NMW) but works fulltime - she looks after the kids but works 3 days a week. But she's always "broke" whilst he is off buying computer games, football kit, you name it - he has money for hobbies and nights out, but my mate never does. Utterly selfish, and his only saving grace is that he is quite a good dad (not perfect - he only does the fun stuff like taking them swimming or buying ice cream in the park - not cleaning up the sick or doing emergency childcare arranging).
Also, I know that you've said he is employed and works hard but that is totally irrelevant, as he has that attitude.
I'd rather be with an unemployed man who knows what teamwork, building a life together really is whilst stoney broke, than a guy with a job with debts and no intention of really contributing his lot.
And I think that's the worst thing. You're not even trying to convince him that he should stump up more of his cash i.e. he already knows it but just doesn't do it - you're having to remind him that he should consider paying more anyway! He doesn't even recognise that he might want to put more in the pot!
THAT fact, along with his debt, makes me see that you will have an uphill struggle here. You're going to have to change his attitude as well as his practical approach to sharing, if you risk your future with him.
And if you do decide to do it, for god's sake, don't cast your DCs financial security in with his and his alone e.g. shared accounts or shared savings - please cordon off anything they need into their own savings accounts for their own needs. You're running a huge financial risk to your own DCs by getting into bed with this man, in more ways than one.