Really fussy dwarf hamster.

[WhiteFire]

I have a dwarf hamster, he was a rescue and we are unsure how old he is. He is so fussy it's untrue. I know most hamsters will leave certain bits of their food, but he leaves pretty much all of it.

So far we have tried:
Harry Hamster Tasty Mix - Leaves the majority of this.
Vitakraft Menu Vital - Eats a bit more but still not great.
Wild Harvest Nature's Nibbles (seed sticks) Will eat it but it stays there for days and days before he finishes it.
Vitakraft Drops - Had one nibble of one, never to be repeated.
Vet IQ Nibblots - will eat these, but not overly enthused by them.
Hazel Hamster Lovelies - If he's in the mood.
Trail Mix - Leaves that. (Raisins, dried banana etc)

He doesn't even do anything with the monkey nuts.

He will eat fresh sweetcorn (dried is rejected) and raw carrot.

Any more ideas what I can try? There is a possibility he survived (abandoned for weeks) by eating his cage mate but I don't want to go there.

Thank you.