To an extent it will depend on the tenancy.
If this is a property he is renting, rather than a property he owns then I would expect that the person on the tenancy agreement has to be the person that is living there. If he is not then he is effectively subletting to you - regardless of who's paying - which is normally not allowed and therefore the actual landlord could throw you out at any time for breach. Of course as long as they're being paid in full and on time they may not care to do so, but it could cause issues if there's ever any problems with the property since you'll have invalidated the lease from day one.
I would strongly advise both you and him to read and understand the lease very carefully before reaching a decision and in any case have a separate, written agreement between yourselves setting out the basics of how the arrangement is going to work - it doesn't have to be anything complicated, just enough to give you both peace of mind and provide backup should it be needed.