Is he offering to pay your rent, or is this a flat he owns and is saying he'll let you stay there for free?
In either scenario, you need to make sure you have a proper tenancy agreement in your own name.
If the former, just say to him you'll need XXXX per month to cover your rent and bills, and you'll be happy to meet him X times a week in return. I would suggest he pays you via bank transfer.
If he owns the flat, he needs to be specific about what's included (and that needs to be laid out in the tenancy agreement.) Is he also going to pay your water, electric, gas, council tax, internet, etc?
If you don't have a tenancy agreement, then he can at any time claim you are a lodger and basically make you homeless. If you have a tenancy agreement then he'd have to go down the section 21 route which would give you a minimum of 2 months' notice to leave.
I would personally not go down the second route, it leaves you way too exposed. In case of the first scenario, I'd want to make sure I still have enough coming in every month to be able to pay my rent and bills, because he will end the arrangement at some point.