Hamster isn’t eating

Hi ! My Russian dwarf hamster is a year old and he’s always been fussy with food. We’ve tried him on different mueslis we had recommended , nuggets etc and we’ve only found one type he’d eat. Even with this muesli he likes to dig through and only pick out certain bits. The bits he does like he REALLY likes.

We’ve tried to leave it but eventually realised it was cater to his whims or he’ll starve.

Recently I’ve been putting his food down and he comes to his bowl ready to eat, digs through it and doesn’t take any of it. He’s been searching a lot , nothing has changed from the bag of food and I’ve inferred he does have an appetite since he is digging around for something. I’ve tried giving him more of the bits he likes and he won’t touch it. He also won’t eat any fresh veggies (ever) , or the little hamster biscuits (never loved them but would eat sometimes).

Does anyone know of any food that might be universally likeable to him? He’s been a little less active but his toilet routine hasn’t changed, I don’t know whether to be concerned or not.