We have been in the worst drought on record in the Seattle area this spring and summer, and that rain we had on Saturday was the first serious rainstorm in about six months! Really, it doesn’t rain that much here overall. A lot of rainy days are just gray and misty. If your son can handle Vermont winter weather, I’m sure he can handle Seattle!
We just thought it was funny that of all the days for it to rain, it was the day that he needed to move. He did a study abroad in Ireland Spring semester last year and thought that the weather in Dublin seemed comparable to that -lots of overcast days. He didn’t mind that either. I have to say every time we talk to him he sounds happy! He’s done at least one meet-up group and then ended up going out with some of those people another night. He found a college friend that just moved to the area too and also got a place in Capitol Hill. He’s happy so I’m happy too. 
It hardly ever gets below freezing here, but when snow falls, you have to be ready for the Armageddon. With all these hills, navigating the city’s sleek streets becomes very difficult. Glad he does not have a long commute and is enjoying his new location.