I have to be completely honest, I think that, especially given the pressure of being a 'kid' these days, this sounds all very innocent and sweet - you said yourself, the messages between them are very sweet, and he has respected her boundaries in terms of a 'kiss' - so credit to you, it sounds like you have raised, and are raising a wonderful son.
If you impose boundaries, ban his phone, ban him from club, stop him going to school early, etc. you are in effect punishing him when, effectively, he hasn't done anything wrong.
You spoke about how your Mother approached things with you when you were younger, this was obviously not the most positive approach, but I suppose we can look at it in terms of being a 'generational' thing, and I daresay she did this to protect you, on many levels, but it had a negative effect - you speak about being bullied because you were too ugly for relationships - yet you have had a relationship, and have children - so the bullies were wrong - but this also had a negative effect on you, all at a time in our lives when you're stuck between not being a child, but not being an adult.
I wonder whether this is unbeknownst playing on your subconscious, and causing you to react the way that you are - you said that you are horrified and pissed off which is probably unreasonable - but I think there are more layers to this than you realise and that it has a lot more to do with you, and your experiences, than those of your Son - he is experiencing something that you didn't at that age; if you let him experience it, then you're going against what your Mum would have done - yet, in effect, if you impose all these bans on him, you're not saying it's 'disgusting', but you are sending out a negative message to him, and are, ultimately, saying it's wrong.
I don't see it as him being deceitful, I see it as his first love, and it all sounds very innocent and sweet (and anything beyond that will have been talked about at school, and perhaps you can have a wee chat with him if this is what you're also worrying about) and I don't see him going to the club, just because of her, they have a shared interest, let him enjoy it.
Don't let on you've read his messages, you'll betray his trust, and then he really may start hiding things from you.
You've been through a lot yourself, in terms of your own relationships, and experiences - don't let them define who you are, or how you treat your son - look at the positives - you've raised this lovely lad, who is a real credit to you, and that's testament to you, as his Mum.
I came across a lovely saying lately:
"we give our children roots, and wings - roots to know where home is, and wings to fly away and exercise what's been taught them"
Don't clip his wings, let him soar.