I know a young boy named [name_m]Leonidas[/name_m] nn [name_m]Leo[/name_m] and I don’t think anyone has ever mentioned it sounding like tendonitis. I think it’s a strong, ancient Roman-sounding name and [name_m]Leo[/name_m] is a great nickname to make it more wearable!
Maybe you could do [name_m]Magnus[/name_m] nn [name_m]Gus[/name_m]? I definitely think that could work; the sounds are right there!
Have you considered [name_m]Sebastian[/name_m] nn [name_m]Seb[/name_m] or [name_m]Baz[/name_m]?
[name_m]Alexander[/name_m] nn Xander/Xan/Zan
[name_m]Nicodemus[/name_m] nn Nick/Nico
[name_m]Demetrius[/name_m] nn Dem/Demi
Maximilian/Maximus nn [name_m]Max[/name_m]
Callixtus nn Cal/Calix