Vetos tend to be a bad thing long term. Some people use it short term as a crutch to help their other partner along. It is better to go with an idea where you agree to work through issues as they arise. Your partner will have to realize that other relationships are not a threat because non-monogamy doesn't force a choice of partners.
I think it is good to talk through possible scenarios. That may help your husband mentally prepare and work out issues before they arise.
Also be aware of NRE and try to damp down your enthusiasm. My wife once asked me what she should do if she noticed me doing things with my girlfriend that I would not have done with her due to NRE. I told her to mention it to me so I could be more aware of what I am doing. My goal is to keep her from having to mention it.