From my understanding your son isn’t your partners but is treated as his. It’s very nice. But does your son have some type of severe abandonment issues or something else? Going by what you posted he must be around 13-14 now. You worry he’d have a meltdown if father isn’t home? I apologize if you already shared what causes these meltdowns.

Personally I don’t give anyone my car. He can rent a car. Don’t give him the key