Okay i have a better understanding of your situation, at this point i would suggest that you talk with him about the possibility of him moving in, but since your relationship is new you just want to know if you are compatible, who knows maybe he is and maybe him and your friend could get along. After all he's probably more threatened by the other guy than you.

Someone, who is a casual boyfriend, that is already making sarcastic, insulting remarks that are insulting to not only you, but two long term friend; even when it is an arangement you say even your family supports so he is actually insulting them too; is a guy that deserves to be dumped. There is a way to express your feelings and work threw fears but his way is not it. He is not fighting fair and it is not going to get better.
The advice to get a signed lease
is excellent.
I will add a working out a written set of House Rules is another excellent idea. If your friend is not up for this discussion before you move in then you may want to consider how equal this home shareing situation is. If you are not paying rent you don't have many rights.
Hopefully you have a working relationship with your family so you can easily move back home if you find out it isn't turning out all rosey posey as few of these mixing boyfriends and best freinds does. If this is a significant friendship in your life I would strongly recommend not mucking it up by mixing friends and business. At the begining of your independent adult life is not the time when you want to loose a BEST FRIEND.
If you are looking for independence in your living arrangement get in a college dorm or a live in job that if you quit you are not stuck oweing for fat lease or your favorite poassesions.

Pacific it sounds like your expectations are a little high, i guess its hard to understand that a guy might have insecurities, and doubts. Being we don't know all the details or the level of this guys tenacity towards the situation i hardly think its fair to assume this guy's sarcasm is overbearing. I'm not saying your wrong i'm just saying we don't know a few things about their relationship and how much the two have communicated about this situation.

and its possible that their level of commitment leaves doubts in each others mind, after all it sounds like a crappy situation for both of them. I know i don't jump in any type of relationship expecting things to go perfectly.