I agree with the advice given that you need to talk to him. There is zero reason why he should be sending sexually explicit messages to anyone who isn't you. I know it won't be an easy topic to broach, but ignoring it isn't what you want to do...I have a history of ignoring signs like that, and each and every time the truth comes to light, it's debilitating. Don't do that to yourself.

Please know you're in my prayers, and I wish you all of my best.

Hugs,
Harley