Brilli,
My husband and I committed to each other months after we started dating, and I knew he had an often-fatal genetic disorder (not ALS; he got that later, though his surgeries for the other disorder went into the double digits during our marriage). So I have a hint of what it's like to have your relationship in many ways defined by illness from the start. Zoohouse (Paulette) does as well, and Music67's partner, and others I'm forgetting -- please chime in, and apologies.
Only you know if this relationship is worth the candle. But I, recognizing how hard your boyfriend's life with ALS will be, would be sure before you commit to it. Read up on ALS, spend time with him, and the right answer for you will be clearer. But we can't tell you "what it's like." That's most defined by the people involved and their feelings for each other, not the disease(s) that they have.
Whatever you decide, you can still be special to each other for the duration. It is not, "commit or walk away" unless one of you wants it to be.