I'm into a guy who's in a relationship

I am into this guy and I think the feeling is mutual. The problem is that he has a girlfriend. So him and I used to be very close and we had strong feelings for eachother, but we were both so immature that we never told eachother(but we both knew). Eventually I got tired of it never going anywhere and told him in quite a rude way, wich ended in us not speaking for a while.

Eventually he moved on and got a girlfriend, and they have been together for 2 years, I think. We still maintained contact over the years. And now we are talking again, and I really enjoy it.

I have also heard that his relationship is not going too well. I have heard that he doesn't want to have sex with his girlfriend anymore and that she initiated a threesome just to get some action, and he didn't really care.

I was at a party a couple of weeks ago, and him and his girlfriend were there. They were fighting because he didn't want to go home and didn't want to have sex with her, wich led to her going home before he did. As the night progressed I decided to walk home(wich is about 2.4 miles), and he asked me to wait 5 minutes so he could walk with me(he's house was right nearby). And as we reached his house he asked "would you like me to walk you home?" and I said "Yes, thanks". He said he would only walk a little bit, but ended ut walking the whole way. And as we were walking we talked the whole way about stupid things.

Now we're still talking and I think we're both kind of into eachother, but I'm not sure if I should act upon it, considering he has a girlfriend. Any advice?

Also, sorry that the story is so cluttered.