While I understand all the "shared pot" suggestions, I can't help but feel if the sexes were reversed, the advice would differ.
That said, his proposal that leaves you out of pocket, and him better off, is grossly unfair.
Perhaps a different way to do it would be to work out all your finances as they stand now, and work out all the costs if you moved in together. (Remove personal bills from this, he shouldn't be paying for your mobile, you shouldn't be paying for his debt) Work out exactly how much money is being saved overall by living together and split it 50/50.
So, plucking numbers out of the air, if your rent, bills, food came to 1000, and you have 2000 coming in every month. After personal bills, child related expenses, you have 200 leftover.
His rent, bills, food came to 2000 and he has 5000 coming in. After personal bills , child related expenses, and debt repayment etc, he has 1000 left over.
7000 in,
3000 bills,
2000 his personal bills
800 your personal bills
1200 left.
You move in together, your income goes down to 1500, your shared bills up to 2500, and I'm assuming his personal bills will decrease a bit, say to 1500.
6500 in
2500 Bills
1500 his personal bills
800 your personal bills
1700 left
That's a JOINT saving of £500, so you should BOTH be better off by £250.
Which would mean that your figures are
1500 in
800 personal bills
450 left over (200 + 250)
250 joint bills contribution
His figures are
5000 in
1500 personal bills
1250 left over (1000 + 250)
2250 joint bills contribution
That way you're both benefitting equally from the merger.
If he argues that in the above scenario, he is paying much more, you're actually contributing £750, and him £1500, because your reduction in income (no UC/child benefit) is subsidising his reduction in outgoings (I'm assuming less maintainence to pay )