Lingering Feelings for a Straight Guy

So this is a bit lengthy, but I've become desperate for help. >.<

I am an 18 year old guy who likes my male best friend of the same age. We've been friends all throughout high school and the feelings have been around all four years. I am aware of the term "limerence" and sometimes I fear that it fits the situation.
The thing is, he claims he's straight. And I don't consider myself gay, just more like in some weird grey area of sexuality and my openness towards the same gender isn't public at all. We're both generally closed off people emotionally, so it's hard to read our relationship and if it leans towards anything hopeful.
We've both have had girlfriends and have had sex with other females, if that means anything.
I get mixed signals from him though. For example, he does stay overnight at my house and vice versa and we share the same bed. It's even gone as far as cuddling and spooning. He's also told me he sleeps better whenever he shares a bed with me. There are more instances of physicl affection, but that one is the most notable. Another thing is whenever one of us gets unwanted attention from unattractive or obnoxious girls, it's understood that we'll act as if we're in a relationship together to get her to leave and then get a laugh out of it.
But there's things that he's done that makes me doubt he may feel the same way. His twin sister is a lesbian and we often make fun of her by calling her car "The Lezmobile". I don't do it because she's gay, it's because she's a bitch. But I don't know his motive. His family also is generally conservative and homophobic.
It's just gotten to the point where it's frustrating to keep this secret, but I don't want to mess up our friendship. We go to the same college and we actually do have plans to get an apartment together once school starts up.

Sorry it's such a long post, I just need some advice on how to approach it. My biggest fear is missing out on something because I didn't have the courage to act on my feelings.