Though an MRI wouldn't hurt, as roller said, in the wrong hands it is not good.
I would start with the PT, make sure the physical therapist is well trained in spine conditions on all levels and let you son have the physical therapy evaluation.
Once that evaluation is done, get the opinion from the physical therapist as to what he/she believes is going on and what that opinion is for decreasing the pain and full recovery.
It very well could be that because your son hasn't been using those muscles for some time now, that they are very weak and weak muscles can cause pain, increase pain and be the cause of what your son is feeling.
Once those muscles have increased strength, you should be able to get a good idea if the PT is decreasing his pain and there is steady improvement.
If the PT doesn't think that she/he can help or PT will help much, OR if after several weeks there is no improvement, I would then go back to the spine surgeon and request a further evaluation, MRI and see what shows.
IF for some reason, surgery is suggested at that point, then it is time to get several other opinions before heading in the direction of surgery.
Good luck
Fran
