I have just started riding my stallion 
  again..after 4 years of not being ridden.  He has always 'mouthed' 
  things---puts anything and everything in his mouth.. ie grabs the reins, the 
  whip handle, the lead rope, the cavesson... all of which make it awkward and 
  slow down the tacking up process.  
   
  Hi Mary-Ann
  Others have posted and I basically agree with 
  what they say.  Don't have years have experience but enough with working 
  endurance stallions to know that one that doesn't respect his handler is a 
  liability and 'mouthing' is an expression of this lack of 
respect.
  At his age its a problem as you're going have to 
  be really strict if you want him to change his habits, but this will confuse 
  him and you might yourself forever smacking him.  Gesa's right though he 
  needs to discover that this behaviour is uncomfortable for him before he'll 
  stop.
  I have a stallion at home, same age as your boy, 
  who does similar but only when I catch him up in the field.  I have 
  established a 'tolerance' level for myself with his behaviour and so mostly I 
  just ignore however from time to time he will grab my hand instead of the 
  headcollar then he will get a serious clout.
  I also have a Polish stallion, born and raised in 
  Poland at one of the State studs.  They don't mess around there and this 
  horse is the opposite of the other one, very nervy and if you hit him you 
  can't get near him for a week.  If he feels the need he will bite the 
  door of his stable but he has never in 6 years tried to bite or kick any of 
  the humans who deal with him.  So it largely depends on the 
  upbringing.
  For future reference colts need to be handled 
  really firmly from the first and correction needs to be instantaneous (not 
  always easy),it needs to hurt to be effective, (watch how mares deal with 
  their offspring) and it should be minimal, just enough, not endless 
  abuse.  Whats cute at 6months is a pain at 2yrs when they're climbing all 
  over you.
  You might see what John Lyons or Pat Parelli have 
  to say about corrective behaviour in an older horse.  Whilst not a big 
  believer in "horse whisperers" if you sift through the mistique there are 
  often some gems to be found.
  Hope this helps
  Heather Hamper
  France
   
  PS  Stallions need to work as well.  
  Work occupies their minds!