The plastic in the bucket may be toxic to him. Put him back in the tank, and if it smells, clean it. That's part of turtle ownership. I had a turtle for a long time, in a tank, and it had to be cleaned every 3 days just about, but that's the responsibility part of pet ownership. If it was only being cleaned once a month, that's why it stank, and you will have to clean that bucket about EVERY day to get rid of the smell.
As for not eating, are you sure he's not eating or do you just not see him eat right away? The turtle could be too cold, put him in the tank and pick your preferred way of providing heat (heat lamp, lounging rock, etc) and make sure that's not too hot for it.
It may also be the stress of being moved from its nice home to a bucket.
Water sanitation may also be an issue. With no cover on the tank, chemicals may accidentally get in the water, and also, without a filter, it's not as clean.
Try other foods too, and see if he likes them.
If he still will not eat after you do these things, take him to a vet to find out what's wrong with him.