Yes, he'll have to take the theory and practical tests as though he's never had a driver's license.
Don't forget that there is now the separate hazard perception test too.
I don't think he needs his provisional to take lessons. Can anyone else confirm either way?
No, there is no specific legal requirement to have a provisional license to take lessons, so long as you are still within the 12-month grace period of your U.S. license. You must obtain it before taking the hazard/theory/practical tests though, in order to get your name onto the DVLA system.
The driving-school sites which are stating a provisional license is required for lessons are just assuming that they're dealing with a new U.K. driver who has never held any license before, in which case the law
does demand that you obtain a provisional before venturing onto a public highway.