This is a Roots Pet rain coat in size mini, navy blue with yellow trim, as pictured here on Luke.
Not sure of his weight when he was wearing this coat, but I'd guess that he was still under 10lbs. The sizing info I can find for this brand lists 10-13" for the x-small, which is the next size up. I can't find info on the mini. He measures at 15" and the x-small seems to be the right fit for him. He is about 13.5lbs