How should I decide what new course to take up?
The holidays are coming up and I'm torn between choosing German or Italian
Don't let somebody else pick something like a language for you. You should choose one because you love it, or you'll make slow (or no) progress.
Pick the one whose culture interests you the most, the one spoken in the country you want to visit the most, or something else along those lines.
Perhaps German as you are learning Spanish, Italian is related and has similarities to Spanish. German is not, so confusion between the two languages may be reduced. That is just what I think and the way I have approached the same situation. It is also something you should decide for yourself as you will probably have more motivation if you chose the one that you really want to learn.
I would use the same reasoning, but to recommend German instead. I imagine learning two closely related languages at the same time can be quite confusing if there are many words that are similar but slightly different. Unless he is already comfortable with Spanish I would imagine that confusing them can be a real problem.