Do you believe in your system? Do you believe that it is sound and it works?
He came to you to learn from you. Teach him YOUR system.
If he wants to do it "boxing" style, if that works better for him, then he can go train in a boxing gym. Why then train with you?
He may not want to do it boxing style but if he's been boxing for a long while it will be instinctive and not a reflection of how he feels about TKD.