Try a small potato, baked, skin and all with some butter and salt for a snack at bedtime. No protein on it. Just been reading a bit of Potatoes, Not Prozac author's website. Potatoes, when served without protein, increase the body's natural seratonin production in the night.
Another thought, I've been reading on here lately about parents who give their kids melatonin to naturally relax at bedtime.
My other idea, cut the afternoon nap WAY back. My oldest stop napping by two, and never had a problem sleeping at night. My youngest will be up until 10 occasionally when he has a really long nap.
Best,
E.