Rhododendron leaf issues can be hard to diagnose because a) many of the symptoms look remarkably similar, and b) they also do not look the same (have similar symptoms) as the same issues on another plant type.

With that in mind, I suspect your rhody is suffering from something called physiological leaf spot. This is kind of a catchall term used to describe various leaf disorders caused by cultural or environmental stresses rather than a specific disease or insect problem. Rhodies are not well suited to growing in containers and that situation alone could account for the leaf spotting you see. It is not a serious problem and generally does not affect the long term health of the plant although it does detract from its appearance. The best way to fix the problem is to plant the rhody in the ground.

Thanks gardengal! Hmm I guess I'll start looking for a spot in the ground.

Unfortunately, Loderi Group cultivars often develop these patterns.

Apparently, a virus.

https://singtree.com/products/loderi-king-george

I've never heard of this. I'm glad the only Loderi type plant I'm growing is a seedling from a fresh duplication of the original cross. Hopefully this means it will avoid the problem.

Boston might be a little cold for it but I guess the OP knows that.

Thanks guys for the info, interesting note from Singing Tree about this Loderi virus. Yes after some hesitation I think it will have to stay in a pot ... perhaps a bigger one.