Did you check for spider mites or scale? The mites are very tiny under the leaves.

GW only lets you submit one photo at a time per post...you can use Flicker, Photobucket or any one of several photo sharing websites.

As far as your seedling (which just as an FYI) since you planted it from seed, chances are it won't produce the same mango as the fruit you got the seed from...unless the variety is Polyembryonic...I posted the difference between the two types of seeds below. Also the downside is it might take sometimes more than 7 or more years for a seedling to fruit. If I were you I would buy a grafted mango tree one that you like and in your zone you will probably see fruits in a 2 or 3 years.

Do you live near the Ocean? "Might" be salt spray coming from the ocean during windy/breezy days. I use to live a mile and a half from the beach when I lived in Bradenton and my young fruit trees would salt spray damage and it looks similar to the damage on your leaves...but this is just a guess.

Subtrop, has a good idea...check it for bugs? Be careful when you do spray the tree make sure you do it when the sun has set or very early before sunrise...some of the products will burn the leaves if you spray when the temps are very warm.

Good luck!

Here is a link that might be useful: Mango seeds...

Thanks so much for your respones. I had an ambitious plan to graft the tree but I think that is out..
So.... yes, it turns out the tree has some kind of insect on it. I'd say it is a spidermite - tiny, transparent, legs. Thing is, I can't find a single spidermite picture that looks like what I have. Granted it is tiny and I only have a small loop for viewing negatives but these look more like tiny lobsters than what I see on google. Wish I had something to take a picture. They move pretty fast. Also doesn't appear to be that many to have done so much damage.

A side note - I suspect these little critters may be the culprits with other mysterious deaths in my garden...