Thanks to all on the advice for toddler hair trimming.
It was so easy with Jean (now age 7)...we just let her hair keep growing. Sadly, in our society I can't do that with my little boy. He does have a cartoon he loves to watch, others are passing bursts and hit-or-miss, but he doesn't like it enough to distract him from something interesting happening by his head.
That said, I gave him a lollipop, which distracted him for a few seconds, then I put a lock of his trimmed hair in the other hand, which he admired and giggled over long enough for me to finish with the clippers. I even got to swipe at the nape of his neck. After the shower--he prefers showers to baths-- I checked and the sides look mostly even; not bad for a speedy job.
Hopefully he'll have a longer attention span in 6 months when we have to do it again.