I think you can just file a ticket and let them know that you're going to be sharing computers or IPs and they'll make a note of it. :)

Account Issues, Other is probably the best fitted one. :)

If he has a username picked out, give them that too, so that they know.

A lot of people also leave a note saying such on their profile. But honestly, my sister and I both play Subeta and use the same internet connection, and we haven't had problems with it at all. So I wouldn't worry too much.

It's usually better to file a ticket than to leave a note on your profile since we don't always get the chance to see those.