Lombardy is united in his game and controls the rest of Italia. You guys are doing this all wrong.
Your vassal king will likely revolt at some point. Let them, then revoke the kingdom as a traitor and make it a viceroyalty.
- Go to the Kingdom of Lombardy title, look up claimants and invite one to your court. Bribe them if you have to, preferably someone with a strong claim. Being 60 years in, there should be a few by now especially as Lombardy has gone elective.
- Land them. Anywhere. A barony will do.
- Press their claim on Lombardy.
- Presto. Your vassal controls Lombardy and it's now part of your realm. Reform empire after one war.
Edit: just saw your comment that it is elective. Yes, elective makes it really hard to marry someone onto the throne or press weak claims, but it also produces plenty of strong claimants in the former king's children.
It'd be hard to do that. The Lombards are probably Catholic. People of another religion won't usually go to your court.