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It won't do any harm to break them up a bit, you don't have to smash them into small pieces, he will still be wearing his teeth eating the smaller pieces & if he has any seeds large or small in the shell that will help with his teeth too.
It might be worth having another go at introducing a little of another food to see if he prefers it now, when you say you don't want to change his food because he likes it but he isn't eating a fair portion of it then maybe it's the smaller seeds he enjoys which you'll get in a dwarf mix.
Has he ever had a whimzee to chew on?