I was where your partner is. It is a huge punch in the gut to find out your partner is cheating on you, even if it is "just" internet/phone sex. It still hurts just as much as if it were IRL one on one, it is a betrayal of trust. We have discovered that there were underlying issues with our relationship that would trigger it, and once it started it was hard to walk away, even when things started to improve. The urge to go back gets overwhelming.
My suggestion to you would be what I have said to my husband, talk to your partner, tell him what is going on before you do anything. Continue to talk to him and find out the true issues.
My understanding of Polyamory is being able to "love" more than one person, ie intimate personal connection. Is this where you truely think you are going or is the internet sex serving a different purpose?