I couldn't see what his eyes were focused on at first, but then I finally spotted it:
We had been seeing quite a bit of butterfly activity along the trail, and then we were very lucky that he noticed this beautiful green hairstreak basking on a Phacelia leaf.
I didn't have a close-up lens on the camera at the time, but I did the best I could to take a few pictures before the butterfly flew off:
This is a Coastal Bramble Hairstreak, sometimes called a Coastal Green Hairstreak (Callophrys dumetorum). [The taxonomy of this species is confusing, so you might encounter different common names and scientific names depending on your source. I'm just listing what I'm seeing used most frequently right now.]
Coastal Bramble Hairstreaks are known for wonderful blue-green colors on the lower surfaces of their wings.
Their caterpillars feed on perennial buckwheats (Eriogonum spp.) and Deerweed (Acmispon glaber, formerly Lotus scoparius).
The adults fly between February and June, so watch for them if you're in the right habitat at this time of year!
I took one more shot before more hikers came along the trail. I hope this encourages you to watch for this little jewel of a butterfly!