Marriage standoff

I'm stuck and confused on how to move forward. 
The last few weeks my husband (10 happy years and 3 kids) has not been himself. He's exhibiting all the signs of a midlife crisis and textbook saying things that shake my solid ground. He swears it's no one else, we will get through it, he doesn't know "what's wrong with him," and he just needs space. We've been really happy, the couple people envy, for a decade. We don't really have problems. We have an active sex life. We are by all accounts, happy. So when he says that he's been unhappy and doesn't know for how long, it's very confusing to me. He's been distant. 
Well, to top that off, he's stopped answering his phone. He has an hour drive to and from work each day. He used to use that to talk to me, because we do have 3 little boys and finding time is tough. Well he stopped. He no longer answers his phone, or says he didn't hear it ring (bullshit - Bluetooth) or makes other excuses. This sounds minor, but these things coupled together make major differences in how he's acting. 
Last night he went to work and left his iPad behind. I know my husband. He's not a loner. If he's not talking to me, he's talking to someone. I search for a couple hours after the kids go to bed. I find very little. His FT app though has been logged out of (I don't know his passwords) which is really unusual. I find nothing on our bank statements, email, phone bills, or joint accounts. He has a credit card though that I don't have access to. 
He also has a secret email account. I'll be honest, I discovered this gmail account 3 or 4 years ago on our shared computer. He had logged out, but I know the email exists, although I should have written down the name. I figured he needed some privacy for his stupid shit. I didn't give it much thought honestly. My husband has zero time to cheat and we've seriously been so good. I've had no reason to think otherwise. 
So I start getting creative. I go on my own iPad and download all of the suspecting apps, snapchat, twitter, kik, I go on tinder, I stop on the online dating sites we met on a decade ago. I search to make sure he doesn't have secret accounts or profiles. I don't find much. But I also don't know that other email address by name, which would help immensely. My husband rarely makes mistakes. I haven't seen that email on anything for 3-4 years. 
Soi go back to his iPad. On a whim I pull up his App Store. And my heart sinks. 5 texting, calling, and video chat apps just sitting there in the cloud. He deleted them from his iPad, but he's downloaded them. Most recently an app called "Hangouts." It uses a google email to log in, a GOOGLE email. My mind flashes to his secret account and my heart starts pounding. 
I go into his FB history and find he's been searching out a specific woman, by first name, and location only. He's looking at multiple profiles to find her. I feel like I might puke. 
I'll be honest, I don't have anything else. I'll never have copies of the messages or phone calls. I knew in my gut he's been speaking to someone. I know he hasn't physically cheated because he simply doesn't have time. But I know he's starting something emotional. 
We are grown ups. So last night when he got out of work, I confronted him. I told him I knew about the apps. He said he was looking for ways to reduce our bill by using alternative calling/texting options. We are WAY under on data and very few phone calls/messages show up on our bill at all. I call bullshit. I tell him again that I already know and he needs to be honest if we are to move forward. He starts getting defensive and deflecting and trying to make me the bad guy. We've had it pretty good. I'm really calm through this. I told him it wouldn't work to shift blame and I'm not taking it. Defensiveness is not a good sign and I'm looking for honesty. He says he's been honest. He comes home at midnight as normal. Sleeps on the couch. I left for work downtown this morning. He has today and tomorrow off. 
We are in a standoff. He won't admit it. I cannot catch him further. I've asked him for truth so we can move forward and he won't provide it. What now?! How do to save my marriage and get my husband back? It seems we are in the start of this emotional disconnection so I don't think it's too late. We are in a standoff and I don't know how to get out of it. 
Edit: I didn't lay out all of my cards, but I laid out a few last night. I told him I knew about the secret email account. He was silent when I told him I've known for years. He's aware. 
I've handled this well. I didn't lose my shit. I didn't turn into a screaming nutcase or a blubbering idiot. I believe I've caught this very early. I also believe this is fixable. The issue is that I may never know the extent of it, I probably won't. I'm looking for him to recommit to the marriage and to me and the kids and quit the crap before it blows up. I'm hoping he wants the same. I'm at work all day today. I plan on saying nothing. The kids go to bed at 7:30. By 8pm we will be alone in a quiet house and I'll reapproach calmly. I'm fairly certain he won't agree to counseling and timing and money would be huge issues for both. But we have always been able to easily talk through things together. So I'm hoping we can do the same here. But he needs to stop shutting me out and want to fix this. I cannot control his actions though, only my own. I'll know more tonight