I'd be careful. Once you start making your bf's fantasy come alive he may:
(1) Love it and accept that you have control of your body and will not mind if your limits are lower than he hoped. A good result for everyone
or
(2) Bf may be very disappointed if you say you have reached you limit, and it will cause problems.
or
(3) Bf will not be able to cope with the new reality of what was a fantasy to begin with.
or....
Good luck...
GYG