It could be: Hij komt elk jaar?
So, what´s the difference between Elk and Ieder ?
Or what is more common?
It's pretty much exactly like "each" and "every" (respectively) in English. So, not much difference, both are common.
Could you please clarify why they use 'ieder' & not "iedere jaar'?
Jaar is a het word (het jaar). So if it comes after an indefinite article (such as een) or no article at all, then the adjective or modifier does not get the e on the end.