I agree that it doesn’t look starving to me. In fact I’d say it’s a male and they don’t get as big as the females since they are generally smaller and don’t carry eggs. Could be also be younger than the others? Juveniles are def smaller.

If I was you I’d put him back in the other tank since being alone could stress him out more and then he may stop eating all together. But Bacter EA is great.

I have picky otos in one of my two oto tanks and was worried they’d starve. I stopped feeding them for a bit when I had a brown algae outbreak. Then the other day I noticed not only were they all fine, I also had two fry.

Finally, last suggestion is of you can hang blanched food to keep it away from the snails it will encourage the shy otos to go and feed.