Oak leaves and acorns can be toxic if eaten in large quantities. So can water that has had oak leaves soaking in it. You'll also want to make sure the grass isn't being treated with chemicals and pesticides. If he's chewing on but not eating them, he should be ok. But if he does develop symptoms, make sure you let your vet know.
Dogs and Oak Trees - San Angelo Dogs | Examiner.com The dangers of oak trees and acorns to dogs - by Alison Bowler - Helium Keeping Fido Safe this Fall | Dog Tails