OP, it sounds like he was fine with his insecurities as long as you were seeing someone pretty much like him. Deep down somewhere, he may have assumed that since you were seeing men who offered what he could offer, then you would stay with him. Mike offers you something different, something you value, and so suddenly, your husband feels he has competition.
So maybe this helps explain the why. Or not.
However, his behavior is not excusable. Yes, you should do what you can to reassure him of his unique place in your life and your bed. Be willing to talk and listen to him about this. Do what you can to ease his insecurities.
Just because you have nothing to hide does not excuse his controlling behavior of checking your messages and phone. That is unacceptable. The temper tantrums and suddenly nitpicking your every move with Mike is also unacceptable.
It is on him to ultimately address his insecurity. You cannot fix his emotions for him. That is not your job. It is his responsibility to do the emotional work of figuring out why he is so insecure all of a sudden and deal with it. You can help. But not your job. You can reassure him until the world ends, and if he hasn't dealt with the root cause, then it will be like a drop in the ocean. Reassurance is a good thing to do, but it will not fix the issue. He is only one who can ultimately resolve this.
Encourage him to find the root of this problem, tell him you will be on that journey with him as a supportive partner, and reassure him from time to time of his place in your life. But do not accept this behavior. It is not. He needs to put a stop to the childish tantrums and controlling spouse bullshit.
(Oh, and by 'fixing' I do not mean telling you to stop seeing Mike, or other kinky people in the future. That would be more controlling behavior.)