I agree with everyone who suggested treats from you dad. I would first have your dad toss a treat in front of Mr Pooch each time he sees him. Then, with time, once Mr Pooch stops barking and seems less hesitant, have your dad offer a treat to him by hand. I think if he's a rescue, he's probably a lot more sensitive, especially since you don't know his background and what he's experienced. But with lots of patience, I'm sure he'll turn around.
Just to give you an idea...
Scottie is a suuuuper paranoid dog, ever since I brought him home at 5 months. He was fearful of all men. Whenever we visited my fiance's home, he'd bark non-stop at fiance's dad. We visited often, but the barking still persisted. His dad loves animals, so each time, he was still friendly to Scottie. I didn't want to bother with getting his dad to help train Scottie to stop barking, since it seemed like a hassle when we were just visitors. It took Scottie
an entire year before he discovered that his dad was safe and actually loved Scottie a lot! Now, he's super loyal and is excited every time he sees him.
With my brother, Scottie also barked a lot. I got him to give Scottie a treat each time we saw him. But what Scottie almost loves more than treats is a really good massage. I told my entire family this. They've all given him a good back massage and Scottie has grown to love them all.
Anyway, Scottie is just a little paranoid boy. But he was easier to manage as he wasn't a rescue. My long-winded point is just that it takes time, some take longer than others. But know that Mr Pooch will eventually know your dad is 'safe' if you do the treat thing.
Just keep at it!