One other suggestion - if it isn't chewing, specifically, that you need, but something for them to DO - try either a Buster Cube or Molecuball (my preference is the Molecuball). These treat dispensers can be filled with dry or slightly moist treats, they have to roll them around to get bits of treats to pop out. It keeps Guinness occupied for awhile when I need him to let me work. When he was younger we fed him one of his meals from the Molecuball instead of in his food dish, just for a fun activity for him to do.