weird mouth thing

Rio has been checked by the vet twice this month and had his teeth floated(he's an 8 yo paint). When he is in his stall he does a strange chewing thing and makes funny faces and sticks his tonge out... the vet says nothing is wrong with him, is this a weird boredom habit? If I rub the side of his face he'll start doing it(he likes his face rubbed). I wish I could let him out in the fields but they are currently flooded, so when he's not being exercised or groomed he has to be in the stall.