Is it easier for my son to fly on british pport or american pport, wondering which one to apply for?

My 2 year old was born in USA but has both British and American birth certificates as I am English but am a permanent resident of America, my husband is an American citizen. Can my son leave the US to visit England on a British pport or do I need to apply for an American pport for him? The process of British pport seems more straightforward than the process for American pport.