All I can think of, you may have tried. To "desensitize" him to being left alone.

Put him in the kennel, leave the room, return, let him out. Maybe a treat for reward. Do that a lot.

Next step, leave the house. Just stand outdoors. Return, let him out, treat. Do that a lot.

Next go out, get in the car and start it. Do not move it. Stop it, return, let him out, treat. Repeat a lot.

Next, drive car around the block.
Next, drive car, wait 5 minutes, 10, return.

If at any point you return and he has been in panic mode, then you moved to the next step before he was ready for it. Back up and redo the prior step.

Is there any alternative to him being alone? Anyone in the extended family he can be left with during the day time? Just another thought.