I'm pretty sure we have all had bad predictive text day or put our foot in it with a comment.
He's got a friend, this is good thing like @YouZirName said. We all need support and vent a bit ( yeah probably about how you did or didn't do something, how he feels about your mum, ideas for your birthday, school issues).
He may have a crush, don't we all from time to time. Taking time off to chaperone, sending messages in his phone without his permission wtf! As soon as you start this type of behaviour of accusations, possession and control then he's going to start to be more isolated and see alternatives as more attractive.
May be take what Mumervine said to heart, but see the lesson in it. If you loose faith, accuse, isolate and belittle your partner, then leave him are you really surprised that he then goes and sleeps with his supportive crush.
I would agree with @Bobbotgegrinch that snooping and being possessive is not likely to help anyone's cause. Even if he doesn't feel this is a huge breach of trust. Let's start somewhere else Is he a good guy? Does he treat you well?
Marriage is like owning a house, the buying it is the easy part, looking after it takes a little work everyday. If you stop maintaining it soon the clogged guttering turns into damp and the dripping tap in the bathroom is a new kitchen.
You don't own him. Take time off for sure, but spend with your husband and do something fun may be find a few days away for the August bank hol, remind him of why you work well together. May be he will start to tell you about his friend, and how you/your close friend/sister/mum did something that really bugs him.
I think half of the people on MN are separated/divorced and seem to be keen to ensure that the other half will soon be.