A little advice please ladies ❤️

Okay so I posted a while ago saying that after me and my partner had our third baby he'd totally changed. No intimacy, no interest really. Going to work early and staying late. Well my sister's just messaged that she knows someone who works with him and he's been trying to get with a girl from his work. Sending her messages and dirty pictures (which he's since deleted so I don't really have any proof) and just generally trying to get with her. I thought that this was probably the case but now that I know I'm not sure what to do... Baby is six months old. We also have two older children.