I always think there's no point in a middle name unless it holds some significance. So if it was me, I'd start there. Do you have any family members (or ancestors) you'd like to honour? Would you like to incorporate a male version of your name, or maybe your maiden name? Someone you admire from the world of literature, science, music, film...? A favourite religious figure? Is there something you'd like his name to mean, something that is special about this pregnancy or baby? A place or song or poem that holds significance?
If start with those questions and see if anything stands out. Michael goes with lots of different names so at least you shouldn't have a big problem making it sound good.