Definitely you need to look roughly at your calorie intake and increase it.
There are lots of great things like quinoa and oats that can add calories and coconut products - milk, cream and oil.
You can cook high calorie veges like sweet potatoes and beetroots, that are not as starchy as potatoes, in lots of coconut or olive oil, flavour up with herbs, and then turn into a mash with extra coconut cream thrown in. Delicious!
Also look at making smoothies and consuming as much as you can - again use non dairy milks, almond milk is great, and blend in fruits like bananas, blueberries and look up other high calorie fruits. Add in peanut butter some dark chocolate and you are packing in the goods.
If you google great recipes for smoothies you will find great ideas that meet your palate.
If you are having any swallowing issues, have you discussed a peg? Honestly, eating can consume a lot of calories if there are breathing and/or swallowing issues. So you can burn up a lot of the calories you are taking in.
Now I will also say that if you really enjoy anything on the list the Functional Medicine doctor said to avoid, I would weigh up what is going to constitute the best quality of life for you. If you have allergies or moral reasons to avoid foods, of course do so. But beware being told that foods are 'bad for the nerves' if the person is not an ALS expert.
And keep asking questions. We have some members here that will probably also give some really good pointers to diet choices they feel help them a lot.
Losing weight, particularly early, really does speed up progression.