I lived in Europe for about 10 years and have a lot of experience with trans-Atlantic flights. Many jet lag symptoms are really related to dehydration, so try to get your child to start drinking extra water at least 24 hours before your flight. Don't just push water on the plane--you need to be drinking extra water well before that. Continue to encourage your child to drink plenty of water on the plane.
Exposure to natural daylight is helpful for resetting body clocks, so when you arrive in Germany, try to spend a lot of outdoor time with your child on the first day.
Often jet lag feels worse the second day after your trip than the first--be prepared for your child to be extra cranky then, and try not to come down too hard on him if he's a little squirrely.