Help!

Hi. I need your advice. I've been dating a Pisces man for a little over 5 months now and we've become best friends. Everything was so effortless so we moved REALLY fast. He brought up looking at a place and brought up the topic of moving in together a couple of times and I went with it. He also talked about marriage and kids with me. All of a sudden he gets scared and steps back. Keep in mind, I NEVER brought up any of this. I just went with it. Later he needs some space to figure things out (if he wants or is ready for this or not). He told me he wasn't ready for how quickly he let me in and he wasn't ready for me to be ready to move in with him, even though he brought it up. I don't understand. He keeps acting like he wants to be with me (forever) and then he fights it.
Now his ex is in the picture... They dated for less than 2 months and she left him with no explanation 7 months ago but they had gotten very close when they were dating... He saw her recently (he was in the hospital and she visited him...) and became even more confused because it turns out he still has feelings for her. Because she left him with no explanation, that door was never closed... He was hurt by her. Now he wants space to decide where she fits in. This just complicated things so much more. He's obviously attracted to her and he has been flirting with her the last few days... I feel like I shouldn't even be asking but should I just give him some space to figure things out? Isn't it likely that he'll just end up dating her or becoming physical/intimate with her while I'm giving him space?