How to handle hearing from an old flame?

In college, I met this incredible guy. We met through mutual friends at some party and neither of us were looking for a relationship at the time. (He actually had a girlfriend when we met, but not for long lol). We hung out many times with mutual friends but after a while it started to be just us. We connected very deeply over the course of the next several months. It was just a friendship though- we didn't date and we never slept together. But he told me he'd never had a relationship like this before, and I agreed. Looking back, I think we might have friend-zoned each other haha. We really were the best of friends. We shared our secrets, we had inside jokes, we were totally ourselves with each other, it was a great friendship! So I was on track to graduate before him, and I did. Before that, we had kind of kicked around the idea of actually dating but knowing that I was moving to a different state after graduation, we decided not to. We remained friends for a while, but distance has a way of either bringing you closer together or pulling you apart. We were tight for a while, but the friendship kind of fizzled out. It didn't die completely. We'd text every now and then, or communicate via Facebook, but even all that stopped after a while. Fast forward to now. It's been about 5 years since graduation and we've both moved on with our lives. All of a sudden, early one morning at work, I get a text from him. He says he misses me, he misses how we used to be, how close we were. I didn't know what to make of it at first, but yeah, I'd been missing him too. So we text back and forth for a little bit, and now it's kind of like we never stopped, like we picked right back up where we left off. But things are a little.... different. He has let it known that he did indeed have deeper feelings for me back then, and he does still have them. Here's the problem- he's got a girlfriend and they even have a very young baby. I don't know if it's that he's unhappy at home that he's suddenly connecting with me or what... I don't really know what to make of all this...