The first tip that can help you in getting your dog to socialized is to get
him into some form of a training program. Even if the training is done at your
own home you will be getting him used to listening to the commands that you are
giving him and he will behave better for you out in public. So even though you
might not consider training to be that important you will want to make sure
that you do have some form of training for your pet because it will make it
easier on you when you take him out in public.
The second tip for socializing your Mastiff and one that I did frequently
was to find a pet store that would allow me to take him into the store with me.
I know that this might seem odd to be doing this, but it does work because he
will be exposed to multiple people typically and it will be on a neutral
location so he will not be as defensive as he would be if someone showed up at
his home.