How do you tell a young man his dream is over?

I train dogs and handlers for the MOD a job i also did for 22 years in the RAF and tomorrow i have to tell a young Airman he has failed the course. What makes it worse is all he wants to do is be a Dog Handler, its all he ever says but sad to say for some reason he just does not gel with any of the Dogs we give him. How do i shatter this boys dreams, any advice?