Maybe it would help you to define whether you are poly or open. The two can be quite different! Also, oftentimes people in established relationships stumble because of assumptions that they both had the same definitions and ideas about what they were embarking upon, when really their visions were different. What does opening up really mean to you and what does it mean to him?
If you have agreed just to be open and pursue others for a primarily sexual connection, there may be a different set of boundaries than if you choose to practice polyamory and pursue other people for more loving, committed relationships. For example, if poly perhaps there would be several "get to know you" dates and an emotional connection established before anyone's hands go down any pants. If you're okay with being just open, then you have to determine in what increments the steps to actual sexual activity should be.
More talking is in order, basically, before any further dalliances take place, I would say.