Matter of fact
Don't make it a big deal, and it won't be to your 5yo.
You might start by creating a photo album of his family: mom, dad, gma, gpa, etc. Include some cousins, aunts, uncles that he may not know along with the 1/2 sibs-just label each photo as you come to it. "this is aunt judy and her kids, these are your 1/2 sisters jill &sue.