I don't think I've ever heard of a U.S. state issuing a license for only automatics. Just because a person may have tested in an automatic, it doesn't generally exclude them from driving a manual vehicle. But, if that is the case, and that restriction is stated on his U.S. license (often listed on the back of the license, and sometimes known as 'Endorsements'), then I would assume that those restrictions would also apply to his driving in the U.K.
If his U.S. state license is truly limited to automatics only, and you only have a manual vehicle, then I'd recommend that he try to get his U.K. license as soon as possible. I believe that a learner's permit does allow you to drive either type of vehicle, though I would imagine that, if he were driving a manual vehicle, he would have to have a licensed adult driver in the car with him, just as if he had no U.S. license.
