Oh, goodness! This is a name style difference! 
Idea #1: You choose first and he chooses middle or vice versa. Then if you have another child, you switch.
Idea #2: Since your names are full names and his are short and sound more nicknamey, what if yours are each given a nickname that he approves?
Leopold, nn [name_u]Lee[/name_u], [name_m]Leo[/name_m], [name_m]Lio[/name_m], [name_m]Lon[/name_m], [name_m]Len[/name_m]
Oslac, nn [name_m]Oz[/name_m], [name_m]Ozzie[/name_m]
[name_m]Julius[/name_m], nn [name_m]Julio[/name_m], [name_u]Jules[/name_u], [name_u]Juli[/name_u]
[name_m]Aslan[/name_m], nn [name_u]Asa[/name_u], [name_u]Lan[/name_u], [name_m]Lanty[/name_m]
[name_m]Nikolai[/name_m], nn [name_u]Nik[/name_u], [name_u]Nikki[/name_u], [name_u]Niko[/name_u], Lai
[name_m]Caspian[/name_m], nn Caz, [name_m]Cas[/name_m], [name_u]Cass[/name_u]
Idea #3: You both go looking for names that include in them [name_u]Rio[/name_u], [name_u]Cruz[/name_u], [name_u]Axel[/name_u], or other names he likes.
Frederico
[name_m]Rodrigo[/name_m]
[name_m]Rigoberto[/name_m]
Adalbarto
etc.
Given how different your styles are, it seems a great option to have the first be from one list and the middle from the other list. There are boys out there who simply don’t want to be [name_m]Leopold[/name_m] and would jump at the chance to be [name_u]Axel[/name_u] just as there are other boys, young Aslans, who would be thrilled to skip [name_u]Rio[/name_u].
Good hunting!