That is the drawback of non-english (non-american) names. Louis, as you already know, is what I consider a French name: "Le baptรชme de Louis" just does not need the apostroph and works fine; okay, at least in France, French oversea departments, parts of Canada, ...
I think here is the issue. "Louis" does not have a silent 'e' but a silent 's'. Now you add the apostroph and the French silent 's' gets pronounced due to english pronounciation habits.
How about "The christening of prince Louis"? This keeps the silent 's'.
And hey, in the german area where I live the ladies' names are leaded by a non-written but spoken 's'. Its just s'Barbara, s'Gertrud and so on if people do their gossip. What I still have to find out is how the native inhabitants pronounce this when the ladie's name starts with an 'S' already: s'Steph? I bet there is an exception to the rule.