should I be worried? (sorry this may end up long)

I'm dating the guy I've had a crush on since sophomore year in high school, we are now freshman in college. Prior to us dating he was with another girl...the entire time they were together he was sliding in to my DM and sending me snaps of his penis or anytime I would post something that would kinda show off my body he was message me... now that I'm dating him I can't help but think he's doing that to other girls. He won't open or reply to any snaps from girls when we are together. He always tilts his phone away from me. He takes his phone with him everywhere, he never leave it in the room with me.I already have issues with this because my ex did stuff like that and it turned out he was cheating on me. I'm not sure how I should go about brining this up to him. I want to avoid accusing him of anything as well as an argument.