Just leave him as he is. It happens now & again. I've had a couple of coaches who only want to be coaches.
To save time, look at the board limit first. Let's say you've got 2 coaches that don't want to be assistant managers and a limit of 8 coaches. That gives you 6 slots to play with. Offer all the coaches or youth coaches you want to keep assistant manager contracts and free up the slots. Hire more coaches, purely as coaches, as mentioned in the original post. When they join, immediately offer them assistant manager contracts.
It's entirely optional if you want to leave them as assistant managers, or offer them contracts back to coaches. I have once or twice though, seen one of the newly contracted coaches to assistant manager become THE assistant, which I didn't want to happen. This is mainly why when I've got the quantity of staff I need, I contract them back as coaches, leaving just one assistant manager.
Just to point out also, I never bother with youth coaches. To my mind they're a waste of a slot, as a normal coach can train both the first team and the youth team players, making a the youth coach role purely cosmetic in my opinion.
I have once or twice had 7x first team coaches and 7x youth team coaches. However, if you've got quite full training schedules with a lot of players, my preference is always 2x coaches for each training area.
For example, I always have 4x fitness coaches. 2x ticked for strength schedule and 2x ticked aerobics schedule.
As they're working with both first team, reserve and youth players, only having 1x coach for each area makes them over-worked and therefore less effective at training. | 
