If you completely control your router and machine you can allow him to set up a reverse ssh tunnel from his machine to yours. Configure your router to allow SSH, create a user account for him on your computer, tell him to run the following:
ssh -R 1234:localhost:22 your_IP
That way he doesn't have to change anything and you can login to his computer by executing:
ssh hisuser@localhost -p 1234
He will have to keep the session open for you to be able to connect. If he closes his terminal, your session will be disconnected. If you want to overcome that you can use a tool like AutoSSH
that will keep the tunnel alive (needs key based authentication to automatically login).
If you want to take it a bit further you can also set up key based authentication between the two computers.
If you don't want to go through the trouble of setting this up, you could also use TeamViewer
Hope that helps.