Onion, try giving him a small spoonful of oats (dry porridge oats are fine but jumbo oats are bigger) and see if he eats those? My syrian loves oats and I feed them over his food daily. Oats are a good source of natural fibre and a slow release carbohydrate food. Dry buckwheat would also go down well. Both can be purchased in Tesco. The Jumbo porridge oats are around £1.22p a bag (black bag in the porridge section in Tesco) and the buckwheat is in the isle that sells gluten free products normally aswell as lentils and other nuts and grains. A small bag is cheap. Buckwheat can be soaked in water overnight aswell to soften it and fed raw the next day, only the texture will be softer.
My syrian is around 14 months now and he`s a strapping lad! Must be the oats!
If you feed Harry Hamster or a mix with large extruded biscuits, hamsters do normally hoard these but rarely eat them. Try giving him less of this and top up with a mix like Burgess Supahamster Harvest, which has smaller biscuits and much more grains and banana pieces in it. Add a few human grade monkey nuts also. Tesco sell these aswell. I find that sometimes adding more interesting ingredients keeps their appetite strong. Fresh veggies of a small size a few times a week like broccoli, carrot, kale, baby sweetcorns (all raw) are good for him.