yeah, what willz said. if he's payin you to record him, then it's up to him to rap on beat. if you record a "crap" song, oh well... tell him to come back when he's ready and can flow on beat. dont be rude, but if that's what it comes to, then let him know.
and, no, i wasnt joking. when i was in middle school band, the band teacher told us to count the rhythm... then play. it works man. it helps to get your sylables right so your flow isnt too choppy or off beat. it's just a suggestion.
bless.