The standard JJB answer to this would be: He's a scumbag. If he cheats on his girlfriend with you, he'll cheat on you with someone else too. Think of the poor gf. Etc.

A more in-depth and honest/harsh/blunt answer from me would be- Yes, he is in to you. He is obviously bored with his three year relationship, the spark is gone, whatever. But he isn't breaking up with her because a)he cares about her as a person and b)they live together and it would be a pain in the ass to find a new apartment and move. I actually kinda admire the dude for being upfront with you about how he has a girlfriend and the whole not sleeping with you thing. Most guys would lie and bang. But that doesn't mean that you're not gonna get your heart broken. He was bored with his old toy after three years of play time, you're the shiny new one. that "spell" he says you have over him will fade just as fast as it bloomed. Literally, one morning you'll wake up and there won't be a text. The end.