Florida has sandy soil that doesn't retain water very well. Potassium deficiency as well. High levels of salt (sodium) in the soil could be making potassium uptake more difficult.

You probably should have used a fertilizer with less nitrogen. On a young mango tree, too much nitrogen can push growth that the plant may not have the nutrients to sustain. that's what I think happened here.

you could try things like liquid kelp, fish emulsion, some organic fertilizer and some synthetic fertilizer, obviously if your tree is burned you should lay off synthetic for a while.

i use those things and my trees seem to be loving it

i use fox farm marine cuisine as my synthetic, but it has organic ingredients as well. Really good stuff and time released

worm castings are awesome! And it's impossible to use too much

Thanks for the feedback! In case it helps anyone else, here's an update on this tree:

I've reached out to my local University of Florida extension office who came out to look at the tree and soil and they think the tree looks to be planted slightly too deep (obviously an oversight on my part) and is showing signs of possibly being too dry and needing some nutrients. At the time of my original post, I had been hesitant to water and fertilize the mango because I read it didn't like too much water and fertilizer but their feedback was that when a tree is fairly young like mine is it should still get some regular watering and nutrients a couple/few times a year.

I'm starting by trying to get the nutrients back to normal with an 8-2-12 or 8-3-9. Here's some information they sent on the type of fertilizer to use in south FL: http://edis.ifas.ufl.edu/ep516. I also pulled the excess soil out from the base of the tree to show the root crown. If the nutrient increase alone doesn't help the tree out, I'm thinking about root pruning and moving the tree up a few inches to be more on a mound which should help with nutrient absorption.

Sorry to revive an old post but looking for an update. My Glenn I planted early this year is looking very similar to your first pic. Lots of new growth but quickly shrivel up and turn brown. Wondering if you had any luck

My Glenn never recovered. Even though I usually do, I didn't pay enough attention when I planted it and when I finally dug it back up I saw obvious signs that the rootball was very bound up when it was planted. The root structure was out-of-whack/gnarled and the tree was also planted slightly too deep. The plant could not properly extract the nutrients from the soil.

For what it's worth, I planted a Keitt at the same time and it's doing wonderfully, loaded with fruit this year. I guess it's a good lesson to pay attention to the root structure when you buy a tree and make sure that if you have one root bound, disperse the roots during planting. Also planting it on a slight mound is ideal, having the root crown a couple inches below the ground can also doom the tree.