"Income from wages," of course, is not the only income taxed by the US (although it may be the only income taxed by France).  If he wants to continue claiming US citizenship, he must comply with US tax law.  That includes filing tax returns, even if after credits and exclusions he does not owe any tax.  

The exclusion for income earned abroad can be lost if not claimed on a timely return.  Or, he must jump through some complicated hoops to assert it.  See Pub 54 and “When You Can Choose the Exclusion.”