Yes, you can send it in a way that requires his signature.
That said, I'm just not quite buying that a 70-year-old, by virtue of being older and retired, has no access to anything electronic. Every 70-year-old retiree I know has and routinely uses email. Most text. All use phones.
The, "He has no access to anything electronic" sounds like a story cooked up by OW and WS to dissuade you from bringing their affair to light. I'd take it with a HUGE grain of salt.
It's very common to be told lies like this. WSs and OPs will go to great lengths to protect themselves.
In my case, the OBS did have a different last name from OW. It took about 20 seconds for someone (here, in Investigative Tips) to help me find his last name. From there, it was a very simple matter to find an email address.
I'd assume that he DOES have email, is on Facebook, and is otherwise accessible online. If you can't locate him, pop into Investigative Tips. Someone will help you.
If you do determine he's not online, there are other ways to tell him. Yes, registered/return receipt is one. But there are also simpler, more direct approaches, such as making a phone call during the workday---when he, ostensibly, is at home, and his wife is at work. You can tell him on the phone, or you can arrange to meet briefly--at which time you can provide evidence, if he'd like it.
Good luck--you're doing the right thing, even though it's hard and it hurts.