Dozer had a nice bushy squirrel tail but he's lost most of his hair, but only on his tail (not on his body at all). What could it be?

My boys are nearly 4 months old. Super close to being released but we planned to keep them a little extra bc Dozer was a bit slow, plus we've had nothing but rain here for weeks with no sign of stopping so we haven't been in any rush. And now I need him to be healthy before he goes.

So about 2 weeks ago I was out of town and my sister took care of the boys for me for 4 days. They've known her since they were 1 week old, but she's not their mom so it was a bit tough for them. She called and told me that Dozer was having soft smelly poop. Not quite diarrhea but not normal either. He had broken a tooth so I'm sure that wasn't helping and I figured she was just giving them too many treats so I said nothing extra from then on: only blocks, milk, and veggies. When we got home a couple days later he had lost weight (they wouldn't eat as much as normal for her) and he was losing his tail hair. Dash has been perfectly fine all along.

Dozer has been acting totally fine since I got home: playing, eating, normal poop, putting weight back on, getting the zoomies, tooth grew back in, etc. So I thought he was fine but his tail is pretty pathetic now.

Is it just going to take more time? Is there anything I can do or that I can give him to help grow it back faster?

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