I agreee with Physiobase.

I would first assess his pelvis (Wikipedia reference-linkSIJ and Pubic Symphysis), L/S and Hip joints.

I would also check his external obliques for their activity since they have a fascial attachment to the contralateral adductors via the pubic symphysis.

As for hip muscle testing, can do them in their standard prone/supine positions - i do. I also do functional testing using everyday activities such as step up/down, walking, squats, etc.

But in my experience, a "stiff" back with groin trouble is more likely to be a pelvis problem.