How dare he ignore your golden nuggets of wisdom?
How do you regain his attention? Increasing loudness is a tactic often employed. After all, you’re presenting an important point don’t want him to miss it.
But, wait! Have you tried using a quieter voice? While it may seem counter-intuitive to get quieter when you want to make a point, this can be an extremely effective tool to use. Your listener will be caught off guard and respond by having to attend better. Instinctively, he will re-establish eye contact and incline his head and body toward you.
Mission accomplished. No yelling necessary.