It looks like you have a couple things going on.   Looks like caterpillar chewing on the leaves.  Caterpillars should only be a short term problem but can be handpicked if you see them.

The smaller, brownish spots are probably caused by a leaf spot fungus.  High humidity and frequent rainfall can help spread fungal spores.   This should not normally seriously damage a shrub but it could recur next season.    Here are some cultural controls you may want to consider:
 - Prune and remove any heavily infested plant material (dispose in trash);
 - Increase air circulation through selective pruning (usually air circulation is not a big problem with rhododendrons);
 - Do not overhead water (possibly use drip irrigation);
 - Clean up plant debris underneath and around the shrub each fall (fungal spores can overwinter in this debris and then be splashed onto leaves by rain and/or sprinklers causing reinfection).

If you want to consider use of fungicides, remember that fungicides are best at preventing fungal disease and most are not effective at eradicating it.  It is best to utilize cultural controls first to try to make the conditions less favorable for fungal infection.  

If you consider a chemical control, please read and follow all label instructions.    Here is some helpful information from the Pacific NW Plant Disease Handbook - https://pnwhandbooks.org/plantdisease/pesticide-articles/fungicides-disease-management-home-landscape