They do need antibiotics usually. I'm so sorry your hammy is unwell. Are you sure it is wet tail? Sometimes they can have a bout of diarrhoea that isn't wet tail - after a change of diet maybe. As the others say, keep him warm and comfortable - make sure he has a dark hide to sleep in (a shoe box with a door cut in would do), and plenty of plain white toilet paper torn into strips for nesting material.
The only natural things I can think of are goldenseal herb, which is supposed to be a natural antibiotic, but I have no idea what a suitable dosage would be for a hamster. Goldenseal can be hard to get and expensive as well - but maybe not in your country? (It's imported to the Uk). Thyme and sage are also antiseptic and sage has a few antibiotic properties as well. If you can't get a fresh leaf then dried herbs might help.
Do you have access to homeopathic medicines? I'm not an expert, but a homeopath could recommend something maybe. Some Vets use homeopathic medicines with animals. They are supposed to be harmless as so diluted.
This alternative vet site says wet tail usually responds to homeopathic treatment but it doesn't say which homeopathic treatment
Cage Mammal Diseases
You could maybe try emailing them? Just looked and they don't seem to have an email address - but it seems they are in the Uk so I'll try giving them a call for you!