eck! Do you think that it is a sensory thing? like he feels the need to have something in his mouth and is seeking the input?
If that is the case then maybe you could get him chewing on stuff instead?
They make these things called "chewy tubes" to chew on. I use them, as they are much safer to chew on then most things
http://www.especialneeds.com/speech-...hewy-toys.html