How to take outdoor portraits under uneven light

My son asked me to take photos of him and his friends before prom at the botanical gardens yesterday. I think overall I did a respectable job, but many photos are unusable because light was falling through leaves before landing on faces, so that faces were blotches of lit and in shadow. Some of the good backgrounds just didn't have an opportunity for the subjects to stand in an evenly-lit place.

Is a fill flash the right tool to use in this case? I didn't have as much time as I would have liked to experiment. On-axis vs. off-axis flash? (I do have the remote trigger.) What starting ISO would you suggest for daylight portraits here?

I half want to call my son and tell him to put his tux back on and have his girlfriend put her dress back on so we can do a second session and I can experiment some more.