I have a Doberman so can probably help.
first, you need to understand the breed a little. They are very affectionate and bond very strongly with their owners. They are not aggressive by nature but do have some natural guard instinct. Very alert and VERY intelligent and gentle.
This makes them **** hard work. Not just because of the breed but becuase of others Disney perception of them so you have to be mindful of that when meeting other **** with poorly trained mutts.
You need to speak to the owner but my guess without seeing it is that it’s attention, play driven. I would suggest trying the following options:
1. divert the mouth from the human to a toy and play and reward
2. Yelp and turn your back arms folded
3. Be consistent
I doubt its dominance – at 18months he’s barely getting his first pubes adolescent wise. Reward good behaviour, exercise and stimulate mentally.
Whatever you do, do not hit or get angry. Ignore the bad reward the good. Make sure you have treats on you. Biting a dog that size is a stupid suggestion, please don’t do it.
I can’t offer more than that without knowing what the base dog is like in its own environment. We’ve invested heavily in ours in terms of socialising and training. He’s the same age and awesome 🙂
There’s nothing more irritating than a Dobe in need of stimulation. They like to be with you and in the thick of it with the family.
Where are you based?
Edit – I cannot stress enough to divert to desirable behaviour and reward with lots of positivity and praise. Be kind. Be consistent. You should get the benefit quickly.
If you haven’t guessed, I’m a huge fan of the breed 😉