That's so sad at 86!
If he was my Dad, I'm thinking he ought not be living alone. There are many dangerous hazards with ALS - especially falling and choking. I would urge you to discuss with him, sooner rather than later, how he feels about having someone stay with him. While he may be doing well at the moment, there is no telling how fast or slow his ALS will progress. Initially, maybe family members and friends can "share the care" in keeping him company and ensuring he's safe and eating well. Ensuring that his home is safe in terms of grab bars, barriers etc. is essential.
This diagnosis is, as you are acutely aware, a very tough one to accept - Doesn't matter how old one may be! Many people get depressed, so keep an eye out for that too.
You have come to the right place for reliable information. We will be here to help support you as you navigate this new reality. I am very sorry you have to be here and I hope the second opinion is better news.