I can't tell what he is thinking..?

my brothers best friend comes to my house to play music. through a series of random weird events, he found out we have the same taste in music and I play guitar too. since then, he comes over and wants me to play music with him,and wont leave me alone until i do, and has been joking about how I'm his future wife and even talked about wanting a girlfriend and asked if I wanted to be his "girly friend." He said it completely seriously, too. Wasnt giggling abd we hadnt been joking before that. Anyway, I'm 4 years younger so I'm wondering if it's kinda like an "awww" thing to him, not serious. Also, I always and pretty much only attract guys who are 4-5+ years older than me. (I don't really know why)