Go to his office and politely ask to chat. Come at a quiet time when he's not likely to be distracted or rushing off to practice.
People are much more likely to take the time when they see a human face. An email or phone message is easy to ignore. A visit also shows your dedication.
As people above said, be ready to tell him about recent race results, training or something to show your level of fitness. Otherwise, he has no idea if you're a 4:20 or 5:20 miler.
Prepare talking points ("Ive been running XX miles per week for XX months....I recently ran a race in this time...I'm ready to train hard over the summer to make the team in the fall" etc).
If you can, find some guys on the team and talk to them first. Tell them what you want to do and ask for their advice.
Have a plan B. If he blows you off, are you going to train anyway?
Many years ago, I was a walk on and a Div 1 school. I called the coach over the summer and he basically blew me off. He never sent me the team training plan but I continued to train on my own and just showed up on first day with a lot of miles under my belt. He took me and several other walk ons--we just had to commit to do the work. He wasn't going to go out of his way to do an unknown freshman any favors (probably a screening tactic) but when we proved ourselves we earned his respect and he became very generous with his time.
Good luck...and run hard!