Can I be PIC as a PP flying under LSA rules (I do not think he has a current medical). With a CFI in a C150?
Since a C-150 does not qualify as a LSA, the PIC in a C-150 with more than one person aboard must have at least Private Pilot privileges (assuming you're not one of the 250 or so Rec Pilots out there), and that means the PIC must hold a valid Third Class or better medical certificate -- whether or not the other person is a CFI.
I know from earlier discussions that I think you can be PIC without a medical as long as there is a qualified Pilot at the other controls.
No, you can't. You may be able to
log PIC time under the "sole manipulator" clause, but without a medical, you can't
act as the PIC of a C-150. The PIC of a non-LS Airplane must have a current, valid medical.
That said, there's no reason you can't get a flight review from a CFI who doesn't have a medical certificate as long as you have a PP and a current medical, and are still PIC-qualified (i.e., your last flight review hasn't run out). Likewise, if the CFI is fully PIC-qualified (medical and all), s/he can still give you a flight review even if you don't have a current, valid medical.
Also isn't a CFI always PIC while in a plane doing a FR?
No. There is no rule saying that the CFI is automatically PIC unless the trainee isn't PIC-qualified (Student Pilot, no medical, no flight review, etc). While the FAA will generally hold a CFI accountable for anything bad that happens during a training flight, assuming both instructor and trainee are PIC-qualified, the decision as to which of the two has "final authority and responsibility for the operation and safety of the flight" is one for the two pilots involved to make before getting in the plane.
