It seems like your son is on the right track. Self-acceptance and non-avoidance are key elements in controlling one's speech, and those can only be achieved by challenging himself in tough speaking situations (like he does in Toastmasters) as well as everyday situations that he finds difficult. Keep in mind that it's a life-long process and has lots of ups and downs. It is essential that you and your son realize that so that you won't be discouraged when it gets rough. Finding a support network will help keep himself in check in such times. Best of luck to your son in college.