Sertoli cell is activated by the FSH. I don't know the detail as how it is regulated. It is at an early stage of forming spermatozoa (aka sperm). Once spermatogenesis has begun, no more Sertoli cells are created. Thus, if you only find sertoli cells only in the testis, it technically is classified as an early maturation arrest. Have a read on wikipedia. It gives you some basic info about sertoli cells.
If that's so easy to control FSH, most NOA would have been cured. It is not the only hormone that regulates the production of sperms. People with sertoli cell only, maturation arrest etc, have an unknown genetic problem at birth. It is still an active research and no one knows what gene has been misfired during conception or birth. That's what we have been told all along by Drs.
I dont' know about the clomid. Perhaps the Dr just thinks it can help. When we consulted with DrG, he didn't tell DH to take anything. However, his levels were borderline normal (low end T and high end FSH). It is the inhibin B that he trusted more. FSH seems to have anticorrelation with inhibin B value.