Bar chewing can be a result of a number of different things. From boredom, to pent up energy, or too little space/stimulation. You could try adding more toys and a wider variety of the ones you have and see if this helps to curve the behavior. Something else you can do to directly fix the issue is add wooden laundry clips to the area which he chews on! To permanently fix the issue, adding a strip of plexiglass all around the perimeter of the bars (wherever your hamster can reach) will stop him from being able to do this. If he does it when you're nearby, it may be his way of asking to come out of his cage which is why it's important that you do not take him out and reward this behavior as it will only perpetuate it! Hope the problem resolves soon!
(Laundry clips
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https://www.amazon.co.uk/gp/aw/d/B01...=laundry+clips