If your going to keep using your tub, a few times a week you should just encourage him into the tub, and give him lots of treats. Then set him free.lol. Soon enough he will associate the tub with good things. Once he's comfortable, you can try turning the water on (dont get him wet, unless the water is just running on his feet) and give him some treats. And then, set him free. Each time he seems to be ok with a step, move forward by turning the water on, and just rubbing his back with your hands (dry, dont wet them).
I had to do this with my youngest GSD. I tried giving him a bath once and ended up IN the tub with him, and the glass shower doors closed so he couldnt escape. That was not working out for him OR me, so i decided to just start slow, and get him used to being in the tub chillen out, then adding the running water, then just giving him some booty scratches while the water was on, and now when i say its *bathtime* they dont mind going in because they know its a good thing.
Also, like some others here have said, the after bath *tug* game with the towel is their favorite part. So maybe when your done working with him in the tub, you could try playing a quick game of tug with an old towel. (try to rub him with it as if you were drying him) but keep it fun and playful. Then he'll know he has to deal with the bath, but knows playtime with you is right around the corner!
Good Luck!
My dogs are the hose police, if they see me even walk towards it they bark and get all excited. Then they attack the hose, so i doubt an outdoor bath would work in my favor! LOL