Praise be to Allaah.
There is nothing wrong with you paying for his ticket, or with him paying for his ticket himself, as he is insisting on doing so and he is seeking reward from Allaah. So let him pay, and he will also be rewarded for taking care of you, in sha Allah.
You will also be rewarded for paying the rest of the expenses for both of you. And Allaah is the Source of help.