Not necessarily. The onus is on him, as the visitor, to show he has funds to support himself.
So - he'll have to show proof of the invite - that's fine, you're able to help with that.
Then he'll have to show funds. If he's staying at yours, then at least he won't need as much as won't have to pay for accommodation. But as long as he satisfies immigration ...
Accommodation is advised upon arrival at the flight destination.
I see three options:
The voucher is replaced with a voucher for the accommodation upon arrival at the flight destination
The accommodation accept the generic voucher you have now
Airtours make a paid reservation in your ...
You have to guarantee to support them for the duration of their stay, you might be on shakier ground. If you are on benefits, the authorities might consider it unreasonable that you can support a guest for a longer period of time, and might ask for e.g. proof that you have some money saved for this purpose. But that evaluation is done by a completely ...