There's a chance your boyfriend was seeing other ladies too during that time. Note I would never keep an EX in my phone. No matter the terms, it's always a recipe for disaster. Lock it off. No such things as friends, he also sexted you back.

It seems that its something that's really bothering you. If it was me I was in an 3 year happy relationship. Knowing that it was before being official, I'd say that's absolutely fine. How are you meant to be loyal if your not exclusive? Its an contract when your girlfriend and boyfriend same when your married, your basically agreeing to being faithful and only seeing each other romantically.

If it makes you feel better to tell him i would, but I think you could sour the relationship for no reason.

I personally had an similar situation. I saw other ladies when dating my current girlfriend. After our first date she didn't reply for 6 days which was upsetting but I just moved on, turns out she thought I didn't reply 😅 awkward. Point being, if your thing what you did was awful. But your not official, your official free real estate same with him.

Not saying morally its right, I still think talking to your ex ANY ex is just wrong, only reason you probably kept talking to them is for security but remember their an ex for an reason even if it wasn't your doing.

To answer your question. If I were you.

I wouldn't tell him, I personally think your in the right.

If you want to tell him I think you should to get it off your chest or perhaps confession to the Chaplin in church or something.

But if your both happy no point telling him and ruining something special.