Well, IMHO, a true friend will understand your pursuit of success and happiness...a lot of goals change as time goes on, surely he can see that. I was supposed to go to college, get my degree, move out into the country and live happily ever after. After my drinking/dope-smoking days of chasing women and partying, I joined the military!
My point is...be honest with him...tell him that it pains you to make such a decision. Heck, let him read this post! The mere fact that you are seeking advice shows that you do, in fact, care about your relationship with him and that you don't want to hurt his feelings, or his dad's for that matter.
Live life to the fullest...your way...don't sell yourself short if you can achieve greater success.