How to link phone number with iMessage using iphone?

The only way I can contact my mate on iMessage is through his email but he wants to link how phone number but doesn't know how… (He only has an iphone)

In settings you can set it to an email or to your phone number.

1. Tap the web address bar at the top of your iPhone Safari web browser, and press and hold the URL address until you get a the "Select/Select All" options.
2. Press select all and then press "Copy."
3. Close the web browser and then press email, or text, depending on how you want to send the link.
4. Press the "Create New" icon and then tap on the blank space where you would usually type the message you were sending.
5. Press "Paste" and then type in a recipient for your message and press "Send."

Your mates Phone No needs to be in 'your' contacts
and listed as/change to be iPhone not just a mobile/work/home No.

if that does not work,
then add his apple ID as the email address in your contact listing also.