you will have to pay tax on your own, not him. Actually, if it is on hud-1, this will be counted as price reduction or closing cost reduction, no tax on your part.
If it is a cash kick back from the agent under the table, he have to pay tax on his rate. You should not pay any tax on it.
If he pays you and send you a 1099-misc, you need to pay tax on your own, not the agent.