I am sorry to hear you are having a tricky time with your beautiful Tibetan Mastiff. It can be very distressing when you don’t understand why your darling doggie is behaving the way he is.
My suggestion would definitely be to head over to your local vet to have an in depth discussion about his long term behaviour and routine. We certainly don’t want you getting injured by his adolescent nibbling and we also want to make sure he’s a happy boy. These behaviours can be quite complicated. It may be stemming from an underlying separation anxiety or boredom/frustration considering he has had a history of destructive behavior (I assume whilst you are out of the house). However, without more information it is impossible to say.
A few things that you can keep in mind in the interim, is whether he is getting enough exercise, enrichment and training. Tibetan Mastiffs are high energy dogs that often do not cope being alone. Ensuring he has plenty of interaction with with his humans is very important. Playing games (fetch or tug), going for regular long walks/runs to exciting places and giving him tough food dispensing toys are all very stimulating activities for him. Doing this will help with boredom/frustration and will create a more content and relaxed dog.
He is still very young so will need to be trained to understand that mouthing is not acceptable. Taking him to obedience classes would be a great idea and they will have endless training tips for you. Trying to replace your hand with a toy that he is allowed to chew on is a good start. It is important to reward him for chewing the correct thing. This requires time and repetitive consistency for him to understand.
If you are not having success, be aware there are veterinary behaviourists that can help. They are veterinary specialists that can offer custom advice regarding your specific situation. If your little mate is suffering from anxiety, he may potentially need some anti-anxiety medication to help him whilst you do behaviour modification training with him.
I hope this helps you make a plan for your mate. All the very best! Let us know how you go.