The main pronunciation issue I see is what other already mentioned--Leo-nid rather than Leo-need. However, if you name him Leonid it's a virtual goldplated guarantee that he'll go by Leo to friends and the general public, with Leonid perhaps happening within the family. you might get away with the full name until maybe kindergarten, but once school starts it's Leo all the way. Leonard on the other hand; I see a lot of naming forums pushing Leo as a nickname for it, but I suspect that's just theorizing. There isn't a huge number of little Leonards running around. If their parents push Leo as a nickname it would stick, if for no other reason than the popularity of Leo (it is even more popular in Canada than in the U.S.). But to my mind, the natural nicknames for Leonard are Len or Lenny.

So basically, what I'm saying is that if you use Leonid, pronunciation may not matter all that much, because he's going to be known as Leo.