You will have to be very careful about connecting any records you find online to your particular Carpentiers; a very limited set of French records are online, and just because there's a Carpentier/Rozier marriage doesn't make it yours or related to yours in any way.
I would ask the people with trees where those parents' names came from; it may well be that somebody found a particular Louis, decided it was the right one, evidence or not, and everyone else copied without thinking of checking. Sadly all too common.
I know people are trying to help, but what we have here is a French man born about 1757 who fled during the Reign of Terror, bearing a very common name (French equivalent of Carpenter).
A marriage in 1770 with similar names to his supposed parents (who may or may not be correct) or a man in Belgium in 1794 with the same name as his supposed father are not necessarily connected, and just searching for people with similar names in France is, in my opinion, a waste of time until you can find concrete evidence of his origins in other records.
One thing to check - have you seen the original of his marriage in England? Does it say if he was single or a widower?
There is a thesis: Refugees of the French Revolution: Émigrés in London, 1789–1802 (you can find a pdf online), which gives some interesting background but more importantly a list of sources, the main of which is the series T 93 at the National Archives -
French Refugees Relief Committee: Records
http://discovery.nationalarchives.gov.uk/details/r/C13830If he received any sort of support on arrival to England, he may be found somewhere in those records, but somebody would have to spend a significant amount of time sifting through records at Kew to figure it out.
