Hi - retired TA (male) here -
I don't think you can expect great expression or interpretation yet - after all, it's only 4 years or so since he learnt to talk!
Others might disagree with me, but I don't think it matters if a book is REALLY difficult, if he shares the reading with you or some other fluent reader; just let him read the words he can manage, and you 'step in' with the harder words.
I used this approach with 'reluctant' readers, whose confidence benefited from feeling they were reading a difficult book, even though they might only be saying the easiest words. Non-fiction can be good for this, for example, books on dinosaurs, science, geography, history, astronomy, engineering etc, so the child says the words he knows or can sound-out, and you do the difficult words - in the case of dinosaurs, VERY difficult words!
To help with expression (if you MUST be concerned with that at this stage), maybe you could find a really easy play script or drama book, where he could act out the different characters, or give some voices to his favourite teddies or dolls.
Hey! I've just thought of a BRILLIANT book : Anne Fine's "Diary of a Killer Cat", which has children's and adult voices, and the Cat! Super funny and a delightful story!
[ I tried to 'post' this on Friday, but MN seemed to be off-line for a while]