Will the external condom (male) reduce a man’s sensitivity?

Try a few different brands or types to see if that helps. You might want to check out the condoms marketed as “ultra-thin” or “ultra-sensitive.”
Still not working? Try switching to a method you can “forget about” for a while, like an IUD, implant, injectable, ring, or patch. None of these other methods will protect against STIs. So if you want STI protection, try an internal condom (female) instead.
Try a different method: implant, IUD, Patch, Ring, injectable.

References:

1. IPPF and UNFPA. (2010). MYTHS, MISPERCEPTIONS AND FEARS: ADDRESSING CONDOM USE BARRIERS. New York . Retrieved from http://bibliobase.sermais.pt:8008/BiblioNET/Upload/PDF4/002988.pdf