My husband was diabetic before ALS as well. He is quadriplegic now and remains on metformin b.i.d [~8h/16h apart due to his schedule] and Januvia, both of which we crush/dissolve. Interestingly, his last A1C (5.4%) was his best in years, despite becoming a "[Oreo fudge creme] cookie monster," a lifelong love of Ruffles and my cutting his metformin dose by 1/3. His diet, which is nowhere as well-regulated/balanced as the one above, is limited by swallowing difficulty, so raisins and nut butters are out and he won't drink much Boost [we blend our own shakes], but second the cashews; we also shell pistachios (sea salt versions for him). ALS' effects on metabolism are not yet fully characterized and he has many other conditions so YMMV. We don't check glucose and his two major liquids are Diet Coke and coffee. Favorite green veggies are cooked broccoli, brussels sprouts. Our motto is, if he can get it down and wants it, he can have it.
Basically, I would say just keep monitoring like any other condition he started with and make adjustments as needed. With ALS, it's much less likely that he'll die of cardiovascular complications of diabetes per se so you are primarily trying to avoid limb complications, infections, large glucose fluctuations, electrolyte imbalance and so on. You and he will learn what works best as you go, and expect it to change as he progresses.