I am guessing that you are playing live: no midi files.

1-Do not change a single thing. It’s very much like having a guest guitar player siting in. You should not change what you have been doing if it has done the job.
2-Yes, you do make room for his vocals and the keys he sings in.
3-Because you are playing live you can make room for his guitar solo.

If he is a good player with sensitivity he will know enough not to get in the way. (Musicianship)

After a bit of time If it is working out (he fits in) you can start making special arrangements.

When sitting in with a different band the best compliment I got is, “it’s good playing everything we played sounded rehearsed. My job was to fit in and add to. Stardom came latter. (Smile)

John C.