He is 6 months old and still needs to be under strict parental supervision.
When does this subside?
When will be become more trustworthy?
he wont unless you make him. it is all up to you if the dog is allowed to get away with this he will never stop just get worse. what you need to do is anytime he does this he has to be disciplined Always look them in the eyes when disciplining this tells them you are boss. when he does wrong show him what was wrong and tell him bad or no what ever your word is then have his toy in your hand and give it too him and good. as you give it too him make sure he know that he can only chew on and tear up his toys. be strict do not let him get away with this not once. make sure he has his toys and being a mastiff I would get him a big piece of raw hide or bone also. you are just going to have to be strict Good luck