Axie won't eat

I've had both my new axies for about a month now, they weren't in the best condition when I got them, their gills were very short and stubby and not 'fluffy' at all, one had an extra leg and the other was exceptionally skinny...

So far Jake, with his/her extra leg, is doing fine, gills aren't very fluffy still but there's a definite improvement Hoshi is still very underweight, he (I think it's a he, the cloacal region is more prominent but I still think he's very young so can't be sure) Towards his hind legs his body goes concave, which has always been a worry but I figured it was because he was so under weight. When I first got them they would both happily snap at any food I put in their tank, now Hoshi doesn't seem interested at all. I moved them in with my wild type the other day and since then Hoshi won't eat at all, all he has done is poo. I have spotted some large gravel around the bottom of the tank (it's bare at the moment until I get new sand) so I'm thinking he ate an awful lot in the pet shop I got him from. I'm wondering if he may be constipated though and that's why he isn't eating? Near his cloacal region faeces can quite clearly be seen when they are forming and he has had black there for the last few days now and doesn't seem to have passed it.
I moved him to a separate tank last night and gave him bloodworms and an earthworm but he just ignored them both and sat sadly staring out of the tank (he seems to get very depressed when away from Jake...) I've since put him back in today but I'm starting to worry, I've tried everything I can to fatten him up Would a holiday to the fridge be the best bet for him? I put him in about a week ago but he wasn't happy at all.
With regards to the water parameters, I'm currently waiting for my test kit to come in the post but I used the last of my dip strip ones today and all the parameters appear fine, temperature wise it's a little on the warm side, between 20°C and 22°C. I've lifted the lid for now and put a desk fan over the top in the hope it will cool it slightly. I'm just wondering if anyone has any suggestions to help my little fella out. I'd say he's currently about 5 inches (room is being jiggled about so no idea where my tape measure or ruler are at the moment!)

Sorry to hear about your little guy. I would suggest a trip back to the fridge. The cool darkness should help him de-stress. Continue to offer food while he's in but don't be surprised if he doesn't eat. Once you feel he has cleared his obstructions, then try returning him.

If you suspect gravel, put him in a small, clear container and shine a light (torch) through him. If he has stones, you should be able to see them. If you see any large ones, you might want to consult a vet.

If you think it would help, you could post a pic. Definitely get a good set of parameters once you get your set in too. The strips aren't always accurate so I would not rule water quality completely out.

Good luck and keep us posted.

Thanks! He's perked up a tad since being put back into the tank but I think I will deffo put him in the fridge later for a few days, check he's OK. I think the body shape may be down to bad breeding...
I'll try the torch later see what shows up. I would consider the water quality but the other 2 seem fine, madam is just a tad moody now she has 2 room mates in her 4ft tank though lol. Will keep you updated

After doing handstands in the tank little Hoshi has now cleared the blockage There's a few more lumps of gravel dotted about so I will put him back in the fridge at some point, just bought some live bloodworm and he's happily snapping away at them. He doesn't show as much interest in the earthworms though Thanks for your help though