There are a lot of reasons why hamsters chew bars. I'm not sure of all of them, but these are the most common ones:
1. Boredom. It doesn't sound or look like that should be the case with your hamster. I see lots of great toys there!!
2. Small space. Hamsters need a LOT of space, and as much as they are climbers, they are runners even more. If you don't want a tank, there are other cages you can buy off the internet that provide a lot of space.
3. Little interaction. Hamsters need to be taken out of their cage pretty much every day.
4. Stress. Is he a new hamster? That might explain why he chews. Mine did too the first week after I brought her home.
I hope any of that helps