I was going to say the same about changing the batteries on the scales
. Ours stopped being accurate and changing the battery did the trick - they're fine now.
I think best option is a vet check. They can check the teeth at the same time. I know it's upheaval for him and a bit of stress temporarily, but if you can't check the teeth easily it's the best option. It's not easy, and especially with a dwarf I imagine. Some people say - wait till they yawn then have a good look - but sometimes they don't yawn much! I am not keen on scruffing as it can distress them. And you still end up not getting a good look.
You can test for diabetes yourself by buying ketodiastix and dipping them in a bit of his urine. Diabetes can have varying symptoms. I suspect it's his teeth probably. Even if his front teeth look fine, he could have toothache in his back teeth and then they struggle to eat hard food. A year seems a bit young for that but they vary. It can be a time when they start needing some soft food, as well as the regular hamster mix - and possibly the vet could prescribe metacam as well, in case there's something else going on (pain relief and anti inflammatory). Which is probably the best option at this stage.
I just bought linseeds and hemp seeds on amazon.
I get the Linwoods shelled ones as they're soft and fluffy - they just lick them up. The whole seeds might need crunching. Holland and barrett sell these
Linwoods Shelled Hemp, 225g : Amazon.co.uk: Grocery