The chance to follow your dream is one rarely offered. You’d be an idiot not to do it.
Don’t sugar coat it. Just tell him that you ARE doing this, full stop. No excuses, explanations or reasons need to be given.
One thing most guys know instinctively and don’t tell is the more you explain, the more likely you are to be out argued. Ever really tried to argue with a guy? We go monosyllabic, giving you a much harder time and less ammunition. The fact that it drives gals wild with rage is a bonus. The technique works the other way around too.