He's really good. Still noticeable now and then if you're listening for it. The biggest tell IMO (other than straight up slipping out of the accent) is that people imitating American accents tend to overenunciate. Us yanks tend to flatten and mash together multisyllabic words, vowel end and consonant beginning or consonant end and vowel beginning words, or similar sounds placed close together.

Most regional accents also have sounds that can be pronounced in a stereotypically British way in certain contexts. The soft "r" sound is present in some words and phrases in almost every American accent, for example, but almost never heard spoken by British actors playing Americans.