Is this cheating? Should I leave him?

I have been with my boyfriend for 3 years we have a beautiful 8 month old son together and I've always felt like we've had a great relationship I recently found his phone when I searched the Internet history he has belonged to ATLEAST 3 single web sites or hook up sites when I comfronted him he denied everything and it of course went into a huge fight. Im still destroyed my heart hurts so bad what do I do?