There's nothing wrong with getting your betta to flare at all - just as long as you're not getting him to flare for very long periods of time (as in more than a few minutes), it is part of their natural behaviour after all. I seen some recommend it in order to keep their muscle tone up, dunno how valid that is tho.
The only "trick" I know is using a mirror but it's possible he's just a very chilled out guy. I know plenty of people on this forum that say their bettas don't really flare - they all have unique personalities! Maybe after he's more settled in and more confident he'll give you a little show of his manliness