How to get my dog to go outside when I’m leaving the house!?

My 7.y.o. male GSDxHuskey has pretty bad separation anxiety. He used to sit in front of the door so I couldn't leave, but would go outside when I told him. He has escalated to either hiding under the bed, or just sitting/lying down and ignoring me and not going outside when I'm trying to leave the house. It takes a good 15 minutes to get him out, sometimes having to pick him up under his back legs to get him to move. He won't take treats if he's knows im leaving, and is not interested in toys. I've tried exercising, spending more time outside together, he gets his food outside (and will follow instructions to go outside to eat), and we have another female GSD who is outside with him. Our vet has recently prescribed him anxiety meds, but they don't seem to be helping

I'm out of ideas and don't know how to help him! I've ordered a pheromone diffuser but it hasn't arrived yet.

Any ideas or advice greatly appreciated!!