Sorry to welcome you here.
ALS is terminal, so we have supportive therapies to help along the path, but no treatments.
It's very hard watching someone go through this, but maybe your FIL wants to do this on his own terms. Quite a number of PALS refuse interventions/therapies because they do not want to prolong their life to experience more and more decline.
It may serve better if you can sit down with him and ask him openly and honestly how he wants to do all this, and would he like to get palliative care involved to help him move through this disease with as much comfort and dignity as possible. Palliative care will work with him on his terms, they will not try to suggest or force any interventions that will prolong his life.
If you try to convince him that he should be accepting things he doesn't want, you may be seen as trying to take from him the only control he has - which is the right to refuse. At the end of the day it is his body and his disease. It is heart wrenching to watch this happen to someone we love, but it's unimaginable what he is going through!
It may also help you to start seeing a counsellor to discuss how you are dealing internally with watching this disease. I saw a counsellor regularly and it really helped me as a CALS.