This is something that needs to be handled legally. Your current boyfriend could talk to an attorney about his rights to privacy and how to get a restraining order against the former boyfriend.
But the legal issues aren’t the only ones. There is a marked lack of maturity on both your parts.
Instead of dealing directly with the problems in your current relationship, you turned to your ex. It’s not true that you “stopped him before things went too far.” They already had gone way too far. You involved your ex in your problems with your BF. You went to a party with him. You allowed him to touch you intimately. Getting drunk is no excuse for this behavior.
Your current boyfriend is equally immature. He is hurt and angry. But instead of dealing directly with his problems with your relationship, he is reading the texts and repeatedly rekindling his anger.
If the two of you want to save this relationship, it’s way past time to stop acting like jealous kids. If you can’t talk about the situation lovingly and constructively, then by all means get some help. Otherwise, the relationship will inevitable explode, but not before both of you are terribly hurt in the process.
I wish you well.