Unless there's a medical issue involved (incontinence, dementia, etc.), then yup. Never too old to learn! It's actually quite common for dogs of any age to regress with potty training when a major life change happens. This could be because the dog doesn't understand that potty rules apply to the new location, or simply because of stress. I'd just go back to treating him like a puppy - schedule regular potty breaks, go out with him so you can praise and give treats when he potties outside, interrupt him (gently) and move him outside if you catch him going inside (and maybe make those scheduled potty breaks more frequent). No punishment, even scolding, or you can get negative consequences like him finding hidden places to potty where you can't see him, and even damage the bond you're trying to build.
Thank you for taking in a senior needing a home. Things should settle as he gets used to you and your household, and I think taking classes with him is a great way for the two of you to bond! A good force-free, rewards-based class is a ton of fun.