Depends on if he wants the flats to get a bill from utlity company (meaning new and separate MPAN numbers) or, if he will receive the bill for the existing main meter then sub meter the flats and rebill.
If its new metering/MPAN/bill from supplier then you may need to contact the supplier to do this work.
If he just wants to sub meter, then buy some meters and install these (two wires in, two wires out) into the feed to each flat.
This way, the utility company reads the main existing meter, bills the owner, the owner would then read the individual flat meters and bill the tenant. The tenants contact would need to include this.
If there is a comunual hall, door entry, lighting etc, then he may want to meter this too in the latter scenario.
Stuart