What breed of terrier do u keep, what have you done with him so far, too young in my book, but there is plenty for him to do at that age,, get his blood pumping with a bit of ratting,, is he stock proof, if he isnt you will never have peace to work that terrier to his full potential,, a terrier on a lead and only let off the lead to go to ground creates a lazy terrier in my book ,,it also limits were you can work the terrier,, has that left you with enough to get on with.