Betta Fish: I need help

Hello all! I need some help.

There is a beta fish in the office I work at and I'm the only one taking care of him, I don't know much about them but try my best to keep him happy and healthy. He is a pretty happy fish, he's always swimming around and doing cool things when I come up to the tank. He ate well , etc..

Then, I cleaned his tank, now mind you, his tank had gone a while without being cleaned (please don't come for me, I'm a manager and have very busy work days so I don't have much time to do things like that while I am here but had a free morning the other day) and when I took him out and put him in a separate container, he completely changed. He dropped to the bottom of the container and was barely moving, I thought maybe he was just upset about the container ao I cleaned the tank as fast as I could and then put him back and he was still acting the same way. And he has been like this for over a week now. I thought maybe he went into shock and would pass on, but he's hanging on the best he can. I also thought maybe it was shock to the water change , but he was acting like this as soon as he was put into the separate container.

I've also noticed he seems bloated ?? I'm not sure.

Does anyone have any advice as to what I can do with this poor guy or what is even wrong with him? How can I help him? Will he ever get better? I feel so awful for him as he just lays on the bottom of the tank all day.