Fuck. I read this as "with someone is an immediate family member" and then got to " also my SO, and that we're both males?" and had a bit of a waiiiiit what.

I'm going to answer the little details over the big change because big changes requires way more dedication than I can muster at 2am. Big changes are generally awesome though. I get antsy without a big change. Be excited!

That said...

Just go with the flow as much as possible. Most things that you plan for won't be big deals and the things you don't plan for will turn into major issues. Just remember communication is key, anger is bad, don't let your frustrations build up, writing down your emotions and feels is good, yelling or texting them is bad, and this other person has lived successfully without you for their entire life. So even if the way they do things is shocking to you, it obviously hasn't killed them yet. Take that into consideration and then determine how frustrating/harmful/annoying/anger-inducing it is to you and how to deal with it.

Take into consideration the things that you do that others may find strange or disgusting and keep that in mind moving forward. I'm a girl and I know every single one of my SOs has hated when I cut my hair in the bathroom and miss a few (like 20) strands and they're on the counter or flor. It doesn't bother me and I always do a full wet wipedown the next morning after the shower but they hate it. So I try not to do it. Things like that.

Bills/rent/groceries/shared items like TP and paper towels and microwaves - sit down and talk it out. Don't take responsibility for everything and don't let other's take responsibility for everything. Have a good mix. In college we had one girl who got all the common items with her parents before the semester started. She then refused to buy anything for the place ever again and eventually would yell at us for using the microwave because we were out of paper towels. It was a mess. Everything should be fair to protect yourself and your roomates from getting a big head. :P

Also, something that is going to be very new is buying groceries and eating food. Don't eat your roommates food. Ever. It's one of those things. You're probably used to eating whatever food is in the fridge without thinking of it. I promise you this will be one of the destroyers of everything happy and sane in a living situation. Sometime's it's okay if you ask and he explicitly states that it's okay. Explicitly. I just wouldn't ever recommend it until you are really, really solid and have had some practice at this. Even asking can be dangerous because it's hard to say no to someone who just asked to eat your cheese sticks. Resentments. Hatred. Hiding food. I've heard of it all.