Viv,
Could you elaborate on what sites you'll be visiting on your "tour to Normandy"? That may provide an indication of the souvenirs that may be available for your Son. I may be able to offer further tips on places to visit also.
As the others have indicated, many of the Museums offer various types of souvenirs, including books, DVD's, etc. The Airborne Museum at St. Mere Eglise has a good selection of items. One cautionary note on the DVD's however - it's important to ensure these are formatted for use on NTSC video systems, rather than PAL (European) systems. While it is possible to have these converted, the quality is often less than ideal. If you're visiting any of the Canadian beaches, the Juno Beach Centre also has various items for sale, and the same at the Pegasus Bridge Museum.
If your tour doesn't include it, you might want to arrange a visit to the Gun Batteries at Longues sur Mer (just outside Bayeux). The Command Bunker was used in the filming of The Longest Day and the Batteries are fairly intact. The Command Bunker is open, but be sure to take a Flashlight.
He'll also be interested in seeing the famous Church in St. Mere Eglise, where Pvt. Steele is STILL hanging from the Bell Tower (he didn't actually land on the side shown in The Longest Day).
When he visits the American Cemetery above Omaha Beach, he may be interested in visiting the graves of two of the Niland brothers, who were the inspiration for Saving Private Ryan (there were four brothers).
One thing you might want to mention to your Son (if you haven't already) is that the appearance of Omaha Beach and the other sites is significantly different than what he may imagine after seeing The Longest Day or other movies. It's often hard to imagine how different they looked in June 1944. Part of the memorial at Pointe du Hoc is "off limits" due to the fact that the cliff is unstable, but the rest can be visited.
Cheers!