Sorry to hear of your previous losses and congrats on your pregnancy! It must be a very joyful and emotional time for your family and I think your husband's intentions as far as the name's meaning are wonderful. I also prefer Leo to Caden, but I love the idea of finding a name you both love with meaning that inspired your husband. You've gotten some great suggestions already. I especially like Griffith, Gabriel, and Nolan. Here are a few more:
Griffin: Irish form of Griffith. One of my absolute faves!
Gunnar: scandinavian, bold warrior, a figure in Norse legend
Armand: French from German, soldier. LOVE Armand. It's soooo handsome.
Sennett: English, bold in victory
Covy: Irish, victorious
Andreas: original Greek version of Andrew, strong and manly.
Donnelly: Irish, dark, brave one
Best of luck to you!
Caden is too trendy for me, I prefer Leo.
Perhaps a Cad-- name (even one that could be nicknamed Caden) to satiate husband, while giving you something more classic/less trendy trendy?
Caderyn (Welsh: battle ruler)
Cadell (Welsh: battle)
Cadmon (Gaelic: wise warrior
Cadoc (Welsh: battle)
Arcadio / Alcadio (Greek: one born in Arcadia)
And hey, a potential winner: Leocadio, which means "clear, bright" in Greek. You can get to both Leo and Caden from it, and you can wait it out to see which fits (or each call him something different :P)
William and John are such wonderfully classic names, I think Leo goes best with them. Caden feels to trendy (as mentioned before). But if both middle names are family names from your side of the family, it may be time to drop back and punt. Maybe try finding another name that your partner loves that you like more than Caden? I hate to say it, but your answer might be neither. Have him talk about other names he likes and see if something feels better to you than Caden.