betta after illness

Hi there,
My betta was very ill I was told it was dropsy but I now doubt I think he had swimming bladder infection. He was treated with Octozin. Now he's better but since the illness (3 weeks) does not eat. for last 2-3 days he shows some interest when I put flakes but would not eat.
He is kept in ca. 9 litres bowl with plastic plants and what I call 'toys' - shells, rocks etc. I do not have filtration/aeration, use oxygen tablets, I used to change him water every week (100%) I know its not good but the water gets dirty...
I have him for 10 months, he is dark blue - beautiful and has become so lively and friendly!
What can I do to make him eat now, am I rights he would recover better after illness while eating? Pls help.

You need to get a small air pump and filter or at least an air stone and air pump. The aquarium is very small, so the water could be polluted. That would, in turn, stress the Betta's immune system and make him vulnerable to parasites and diseases. A nine litre tank is small enough to require some very rigorous maintenance. Twenty or 30 litres would be easier to care for. If the Betta has a consistently healthy environment, he will be healthy.

Okay I think the decision to get a proper aquarium tank is firm now. You know all these people - they say why do you keep your betta in such a BIG tank! I mean the bowl is large enough to make entertained but I would agree that the water conditions are uncontrolled. By the way how to make him eat again???

He will eat when he is healthy. His tummy is about the size of his eye, so a Betta doesn't eat much. One of his favorite foods is frozen, then well thawed, blood worms. The larger aquarium should have plenty of plants or decorations to break up water turbulence and provide hiding/resting places.