There really isn't much that you can say besides I love you and I'm here for you. I've had 3 friends that were faced with ALS. I hate even mentioning the disease, especially to someone whose friend has contracted it. Sometimes ignorance isn't bliss & being prepared to deal with reality may save a little pain. 1 friend I'd known since childhood & stopped in often & hung out a lot. Great guy, except for being a total health nut, he drove me nuts with the nagging. He told me 1 day & thought he was joking. I noticed no changes at all, 3 weeks later I saw him in the hospital, 2 weeks after that I was carrying his coffin. The other 2 had more time, but the result the same. Just pray & be there for the family
The best thing to say is to encourage him and show him how much you care for him. Give him tips on how to deal with the disease and advice him to seek medical attention to heal the disease. A person with this disease may show symptoms like inability to initiate and control all voluntary movement, weak muscles and slurring of speech among others. It would be good that you support your friend through any difficulties he may face.
Thats so sad. Just be there for him. Theres not much to say. Just try not to treat him differently than usual, at least until his condition requires it. When people get sick friends tend to disappear because illness is hard to watch and deal with and ALS is pretty rough. Try to be the friend who sticks around. His may progress slowly but it may also progress very quickly and you may have a few more years or just a few months left with him.