Keep him on a leash. Require that he follow you where you go, waits until you enter a room before entering. Try to not take anything personally, don't radiate feelings of anger, frustration. Just visualize that you are the leader and he will accept that. When he's in a calm state, quietly praise and rub him. (A carrot to go with the stick of dominance).
It's hard to offer advice. If he's putting teeth on you, shows any signs of aggression, is a breed with a reputation for biting... my advice might escalate things. But, you definitely don't want to be fearful. You have to be seen as calm.
What breed is s/he? How old? What are examples of "strong will?"
I wonder in what ways your wills are clashing? Why do you need him to do something different? If you're just sitting, what do you need him to do? Is it possible to just avoid requiring anything of him and give him his space until the owner returns? What is the reason to put him in a submissive position to you?