Although you can do colour separations in Photoshop, the main difficulty is that you need a perfectly calibrated system to match your monitor to his print.
If you have no experience whatsoever, it is best to buy a good book (the subject is way too complex to handle here) on Photoshop (like Gary Bouton's Inside Photoshop for example) and also one on colour and quality.
The main thing is to work as long as possible in RGB and convert to CMYK at the end because your monitor always displays RGB and can only simulate CMYK.
Some of the topics that you have to look at/ know about:
1/ What kind of press will he use?
2/ What are the limits he uses in the dark and the bright parts of the screen?
3/ What kind of screen does he prefer?
4/ What kind of calibration does he use?
5/ Does he have a computer guided ink density setting?
Normally he has some limits inside which he can play with the densities, but it makes a lot of difference if he uses UCR, GCR or GCR with UCA.