Some suggestions - specifically for care of the little one:
1.) Separate. Just in case his lack of eating is attributable to being "sick."
2.) There are two ways to feed a gecko that won't eat: get a plastic syringe - without the needle (you can find them at Walgreens or Eckerds), and fill with roughly .5 to 1.0 cc's of substance (gatorade, soft dog food, etc). You can either carefully place a tiny amount of "stuff" onto the gecko's snout and wait for the animal to lick it up. However, this usually leads to only tiny amounts being consumed and does require a lot of time. If that doesn't work, holding the gecko in a cupped hand so that his head is sticking out and he can't move his body, gently rub his mouth until he gets really pissed and opens up. Have the syringe ready - and put into his open mouth. Be careful as you dispense the liquid or food from the syringe. If done too forcefully, it might go into his lungs if not swallowed properly. This is also very stressful on the gecko, and you will want to live him alone once this process is complete. Keep careful records of how much he "eats" and when. I would recommend starting to try to feed him 2 - 3 times a day. Especially if he's really thin. Also, after the "force feeding" and after some relaxation time, try putting him in a small plastic carrier with at least two or three crickets that have been dusted with calcium. Leave in there no more than 20 - 30 minutes. Watch carefully - don't want to add any additional undue stress. Over time, hopefully, he will start eating live food again.
2.) The Reptivite is something that can be purchased at any PetCo, PetsMart, any local pet store that sells reptiles, etc. It's very handy to have, it's only about $7 - $10 for a huge packet of powder and comes with very specific instructions. In order to administer correctly, though, you will have to know the weight of the animal.
3.) Other options for "forced substance" - cricket cocktail: $10 worth of crickets, some Ensure, some calcium powder and some Reptivite. Blend until smooth. Take out what you will be feeding for the next day or so - freeze the rest in a standard ice cube tray. Each tiny cube should provide at least two feedings. Can be gently thawed out by placing into a glass and then the glass is placed into warm water. Will also allow you to break off in small amounts only what's needed. Be forewarned, though, my tiny guy really hates this stuff.
4.) VET, VET, VET. I recently spent $120 total on three rescues, none of which are going to make it. But, it was about commitment for me. If at all possible, it's critical to get a fecal sample. In my case, especially with the last little one left, Pablo, the fecal samples have all been too dry or too old. He's getting to the point where he's going so infrequently. So, even if you can't afford to put a lot into the little one, spend the $20 - $30 to get a fecal done. His lack of eating after all could be more than stress induced... and he was free. If you can afford to keep, you can afford the vet bill....
5.) I'm trying to think of what else... like I said, I've recently been to two different vet visits... Mixing the Reptivite with the gatorade was recommended by the vet. And make sure that you are practicing good husbandry - change the towel as soon as he poops, watch the humidity, offer a big enough bowl for him to soak in completely, offer a hide, temps are warm enough, etc.
6.) Know when to let go. In my case, each departure is a tragedy - thank god for my husband's support. I have to be strong enough to not let my emotions get the better of me, though - if Pablo isn't better by Sunday, I will have to put him down. I don't want to, I want to pray for the best, but sometimes, the best isn't going to come. And, it's better to humanely say good bye when necessary, then watch a creature suffer from starvation or from sickness....
I sincerely hope this advice helps. I did not intend to write a dissertaion, but I feel very compelled regarding this subject. I should be studying Calculus right now, instead, I'm painfully upset about my dying little one - no one was able to identify what the hell is wrong with him. And do believe me, I have tried.
So good luck. I would be willing to exchange contact info - in the instance where I could possibly help you. Again, none of this is "professional" advice, just advice from someone who really cares about these creatures.
- D
and Laly 
. A Pair of juveniles.