What do you mean by “talking to” his ex and other women? You don’t mention the substance of these texts. Is it flirting, sexting, or just normal chatting with friends? It’s hard to tell from what you’ve written if he’s being inappropriate or if you’re someone who gets jealous easily or who maybe doesn’t believe people of opposite genders can be friends.
Generally, if you’re ever at a point where you’re asking yourself if you should stay or go, the answer is always leave. People in happy, healthy relationships aren’t asking that question. Nor do they surreptitiously check each other’s phones.