so he's a little chap, probably with a little mouth and a fine muzzle. when the bit is in, next time, see if you can lift up the side of his lip and see how much room ther is above the bit - whether it's touching the palate or whether there's space above it.
it sounds likely that when there is contact on the bit, moving it in the mouth, he just hasn't got room for it, so is putting his tongue over to stop feeling like he has such a mouthful.
try a narrower bit - a french link with a narrow mouthpiece and preferably a lozenge, rather than normal link, which might be too long for a narrow mouth.
i had the same problem with one i backed last year - a 12hh NF mare. she had a tiny mouth and a fat tongue, so with most bits she just didn't have room once there was contact on the bit and it wasn't just sitting there. we use a FL bridoon now, with a shallow link in the middle. she was also happy in a nathe bit, but after she ate 2 of them we had to go with metal. i like the nathe bits as opposed to the happy muth ones - they're a smoother material, so no dragging on the lips which can be a problem with rubber or happy mouth, and also they're narrower in the middle so there is more room in a little gob.