Trust is crucial to a relationship. If you have told him that you don't want him discuss details of your private life with his friends (and really, any partner worthwhile wouldn't even need to be told this), and he told them anyway, that's a problem. You need to explain to him that this is something very personal to you and you do not want it broadcast to the world. If he doesn't agree, it's probably best you move on.
That said, you're 18 years old. You talk as though you have been part of this community for years, but really you're just starting out. Having a diaper fascination or inkling to go back to infancy when you're five doesn't make you ABDL. Most ABDLs don't connect the dots to realize they are ABDL until well into puberty, and DDLG is a relationship power dynamic that I would sincerely pray you had no urge towards at such a young age. The reality is despite your "experience", you are young and new to the give-and-take of a relationship. Relationships can be as messy as your diapers and just as stinky sometimes too. If this one doesn't work out, there will be others down the road. The important thing is you learn from the lessons in this relationship and apply them to the next if that's what it comes to.