well, with this new information you've potentially narrowed your search down to regiments raised/based in Alsace-Lorraine. These are the eastern most provinces of traditional France and the dialect/accent of French spoken here is quite distinctively germanic.
In terms of tracing your ancestor you now know he was in the Grande Armee that entered Russia in 1812 and died there along with approximately 700,000 other Europeans (not just Frenchmen). You also know that he was born in Alsace-Lorraine and was therefore LIKELY to have been serving with a regiment raised in that area.
With this information you should probably try and contact the French Army Museum/Archives based in Vincennes in France. There may be other sources for regimental listings but Vincennes would be a good place to start. Of course they will need to know a name. (I'm presuming you HAVE his name from William's birth certificate.)