He sounds like such a love. How old is Brutus? I'd be willing to gamble that because he is of such giant sized ancestry he was probably treated with less than gentle hands when he was a puppy. Sounds like he may even have been hit and definitely yelled at.
I would encourage you to continue what you are doing. Calm and quiet. No loud, even if happy voice. Just matter of fact. When other family member/friends come over have them just ignore him. When he finally ventures over, no 'good boy' or anything that brings attention to him. Let him size up. If he comes within petting distance, have person continue to talk in normal tone to you and gently, nonchalantly try giving a subtle pat on head or scratch, never looking at him.
I would also play with your other dog without inviting Brutus. Have a little wrestle, toy pull, whatever you do with other dog with lots of happy sounds and enthusiasm. Seeing the other dog having fun and that nothing bad is happening to him will peak his curiosity. Don't involve him right away if he comes to check it out. Let him make the first move to join the fun.
With him being so submissive I can't see that you are having problems needing corrective action such as jumping up or chewing so probably no need to use stern voice with him at this stage.
We acquired a German Shepard years ago who was severely beaten. With TONS of patience she eventually started to trust me, then my mom, then women and finally men. It took about 2 years before my brother could walk by without her running for cover. She lived to be 14yrs and turned into such a suck-up to anyone. You want to rob my house? Come on in, but could you give me a belly rub please.
I think Brutus will come around much sooner as he is already bonding with you. You are doing the right thing, hang in there. The fact that you adopted him and are inquiring about what to do shows he will have a wonderful life in your household.
