That's a very short time for a partner to end a relationship. It just shows even if the two of you reunited, the reunion would be pointless. It would end in another breakup.
Stop following him on Instagram, and block him from you social media accounts. Cease any communication with him and have nothing to do with him anymore.
Your ex is manifesting lack of maturity in a relationship. The extent of his asking his friends to gang up on you and your friend clearly indicates his personality. Also, it shows how he can't contain his jealousy and his controlling or domineering personality. You're better off without him.