Can we see a photo of him? That may help answer some questions about hydration, etc. I would like to compare a new photo to your other photos of him to see if there is a change.
You don't necessarily need the vet if your cham is dehydrated. Looks like he needs a probiotic and increased hydration.
Your guy just recently got off a course of antibiotics. Normally during a course of antibiotics it is best to increase hydration to help wash infection, etc., out of the body. Also, during a course of antibiotics, the good bacteria in the intestinal tract can be disturbed or wiped out. This is quite common, and the result is exactly what your cham appears to be going through based on what you said here. My guess is that he is recovering from some side affects of the antibiotic.
To increase his appetite I would recommend 2 things:
1. Good hydration. Just stay on it and try at least once a day to get him to actually drink while you are watching him do it.
2. Give him a probiotic to replace any good intestinal bacteria that may have been disturbed by the antibiotic. When the good bacteria cultures in his digestive tract are destroyed by antibiotics he will lose his appetite and have difficulty regaining it. He will also have difficulty digesting what he does eat. It is the same as for humans. When humans take a long course of antibiotics it is recommended they consume yogurt or acidopholis pills to replace the good bacteria that the antibiotic wipes out. My guess is that is what is going on with your guy. We keep probiotics on hand for our chams. The one we use is
Don't know if it is available in Sweden. If you cannot find a probiotic such as this I would call your vet's office and ask about one.
Hope this helps. Post a new photo if you can, and let us know if you find a probiotic and how he progresses.