Oh my goodness..A classic combination of a soil that has compacted around the root preventing them from breathing and taking up moisture..Then add to that a lack of direct sunlight and dry warm air inside..

The very first thing I would do is change the mix..Is that possible to do for you?

Can i have a little bit more elaboration? On the specific soil, etc. I can do virtually anything.

There is a soil readily available that other say works, but I have never tried them just yet...Silica here could help you with that if he sees this. Brian can also suggest a mix for you..

What I can tell you is to use a 5.1.1 mix which consists of 5 parts small bark, 1 part peat, one part 'perlite'..Then add t teaspoon of lime per gallon pot..Make sure it's moist when mixing it before you plant in it..

Your goal is to allow those roots to breath in a porous mix asap..Even if you use a 5 parts 'perlite' and one part bark. It has to drain well and be porous...

What do you have access too? What kind of soil mixes? Pine bark? Reti-bark? Some very porous bagged mix?

So you have lots of sun to provide for it in a very warm window?

The best I can do is try to order it from Amazon or go to my local store for this kind of stuff. I don't have this on hand. Can I get a link?
I have lots of Sun for it. Every day, it gets at least 10+ hours on a non-cloudy. The window is quite warm. I would say my house never goes under 75 degrees, especially near the window.

I'm thinking maybe fast and cheapest. I just want to save my little guy. He'll be alright, yes?
And thanks so much.

Ok...What do you have on hand? For a mix? Perlite? Do you have any local Big Box stores? Do you have access to big pots? Bigger than the one you have your tree in now? One that you could easily rest that tree and that pot into it?

Hello Mark. You can get the ingredients for he 5-1-1 mix from Amazon and local garden center.

When repotting into a different kind of
soil mix, remove the old soil mix by dunking the root ball in and out of water.
You may have to use a garden hose to gently force the old soil off.

People on this forum use either the
gritty or 5-1-1 mix. The ingredients for 5-1-1 are easier to find. This is what
I use:

My version of Al Tapla's 5-1-1 Mix

· 5
parts pine bark mulch (remove ½” pieces through 1/2 inch screen)

(Some people use Repti Bark instead of
mulch. This does not require screening but may require more frequent watering.)
Be sure to soak the bark overnight.

· 1
part sphagnum peat

· 1
part perlite (coarse, if you can get it)

· 1 Tb garden
lime /gal 5-1-1 mix (measure the 5-1-1 mix)

· Osmocote
plus (follow manufacturer’s instructions). This is optional.

·
Moisten well and wait at least 2 days before using

PLEASE NOTE: Other people with many
years experience growing citrus in containers do not use the above soil mixes.
For example, Millet (TropicalFruitForum.com) uses 9
parts Miracle Grow Garden Soil (NOT Miracle Grow Potting Soil) to 1 part perlite. The general consensus on
this forum is that you are more likely to avoid root rot using the gritty or
5-1-1 mix.

Thanks Vladimir! I could not find your post for all the money in the world. Perfectly said)

If you find that you choose to use something other than the porous mixes we use and you see that the new mix is till holding too much water, we can fix that when the time comes..

Okay, guys. Thanks for the information. I am off to my local store to scavenge for the 5-1-1 ingredients and if I can't find them, I'll find the 9-to-1 stuff. Thanks so much and for responding so quickly.

Okay, everything is mixed and ready.
My Home Depot sold already moistened and wet-ted cedar mulch. I asked their experts and they said it was fine for a 5-1-1 mix for a lemon tree. I will be replanting soon. I used the 10 gallons of the mulch, two gallons of perlite, two gallons of sphagnum peat, and 14 tbsp. of lime (not lime juice, the rock).
Here is a pic:

Cedar is not pine and does not decompose the way pine does. I don't know what effect that will have. I hope you used garden lime and not plain lime.

No, I used garden lime.

I am sorry you used the advice of the Home Depot "experts". Here is a quote from Al Tapla who developed the 5-1-1 mix:

I can say that I haven't actually used cedar bark/mulch to grow
in, so I can't comment on the practice directly or from actual
experience, but I can say that because the tannins, as well as terpenoid
and phenolic compounds cedar is rich in are known to be allelopathic
(inhibit growth) to many other plants, I have discounted the likelihood
that it would be a suitable choice for me. :-(

Seeing he bought a mulch, I would not worry about the compounds.

I too have never use Ceder Mulch, but I have always used pine or fir bark..Even Arciada works great too)

Chris, why not worry because it is a mulch?

Already decomposed.

http://gardening.wsu.edu/do-cedar-mulches-kill-other-plants/

https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/aleopathic-wood-chips.pdf

You mean Al Talpa, the guru of container growth media, is wrong? How could †hat be?????? Actually, Al did not say definitively that that cedar mulch would cause problems only that he ".....discounted the likelihood
that it would be a suitable choice........"

Thanks for your reply, Chris. I learned something.

I can't say he is wrong. Maybe what he said has to do with shredded cedar (as opposed to mulch)? I am not sure. I honestly have not been on gardenweb long enough to have read everything here. Cedar, and its related compounds, is something I am well versed on, as they are relative to my profession (veterinary medicine).

I am not botanist, nor do I play one on TV. ;-)