I wouldn't worry about it. If the snake is acting normal and feeding, count your blessings. No need to try and open the snakes mouth just to see its tongue which you wont see if you open the mouth anyways. Just watch closely when you feed him, I'm sure he is flicking it. My Bp doesn't flick her tongue that much either, she is just content to cruise around. Both my boas will flick their tongues at anything. they especially love to do rapid fire flicking on my hand as I pet them on their heads.