A lot of good answers, but as your probability of staying there is low, I'd advise another approach to my daughter, if she was in that kind of situation. It's rather desperate anyways, so maybe it's time to do something desperate too.

You say you are non-confrontational. He is bullying(and that's probably a weak word) you exactly because of that. He sees you as a prey, not a fellow human being. He's the kind of predator who does see the world only in terms of power. You have to prove that you do not fear him(even if you do). You have to confront him. You have tou counter-attack him. Vith verbal violence. Of course no physical violence(then you would and only you would be wrong), but verbal violence. Just once. Use the surprise effect. Be ready for the next time he is bullying you. and suddenly, without a warning, explose and throw him a flow of insults, as ugly as possible.

Yeah, I know. This might get you fired. But if you don't solve the situation quickly, your days within the firm as numbered anyways, as other people said. So be prepared to leave, and then send him back to the school bench he should never have left.

P.S.  : one of the risks of that kind of tactics, though, is that you risk to like it. And it should be used only in desperate circumstances, so it's better to be reluctant to use them. And you have to assume your behaviour if the boss asks you. And you might be fired for it, if your boss does give the priority to his buddy. It's a high-risk strategy. But in such a small startup, with I guess not much formal process, standard advises like "write down everything" and "call HR", which are very sound in a big corporate environment, are not even giving you as much chances of success, I fear.

TL;DR  : situation is desperate, time to try desperate things, and be harshly confrontational, once in your life.

A lot of good answers, but as your probability of staying there is low, I'd advise another approach to my daughter, if she was in that kind of situation. It's rather desperate anyway, so maybe it's time to do something desperate too.

You say you are non-confrontational. He is bullying  (and that's probably a weak word) you exactly because of that. He sees you as a prey, not a fellow human being. He's the kind of predator who does see the world only in terms of power. 

You have to prove that you do not fear him  (even if you do). You have to confront him. You have to counter-attack him. With verbal violence. Of course no physical violence  (then you would and only you would be wrong), but verbal violence. Just once. Use the surprise effect. Be ready for the next time he is bullying you. And suddenly, without a warning, explode and throw him a flow of insults, as ugly as possible.

Yeah, I know. This might get you fired. But if you don't solve the situation quickly, your days within the firm as numbered anyway, as other people said. So be prepared to leave, and then send him back to the school bench he should never have left.

P.S.: One of the risks of that kind of tactics, though, is that you risk to like it. And it should be used only in desperate circumstances, so it's better to be reluctant to use them. And you have to assume your behaviour if the boss asks you. 

And you might be fired for it, if your boss does give the priority to his buddy. It's a high-risk strategy. But in such a small startup, with, I guess, not much formal process, standard advises like "write down everything" and "call HR", which are very sound in a big corporate environment, are not even giving you as much chances of success, I fear.

TL;DR: The situation is desperate, and it is time to try desperate things, and be harshly confrontational, once in your life.