Firstly, everyone forgets things now and again. It may help if you make a written record of what you have been asked to do, then you can both refer to it. If you have a ticketing system you may be able to leverage that to keep the records. It will also provide a mechanism for reporting progress and completion, and keeping a record of changes to the requirement.
Secondly, if the stories are preventing you from getting your work done in a timely manner, you need to work out a strategy of interrupting the stories in a polite way. Perhaps suggesting that you could discuss the matter later, as you have a deadline to meet with , or see below.
Thirdly, if there are procedures about needing tickets for work, you need to follow them. this also ties into the first point above. The more you use the ticketing system, the more you will be aware of the need to create tickets.
On a Management course I went on, years ago, I was taught that there are three types of interaction.
Adult ↔ Adult
Adult ↔ Child (of which Parent ↔ Child is a special case)
Child ↔ Child
In a work environment you must strive to keep the interactions at the first. When things degenerate, you end up in the third, which is basically an argument. You seem to be frequently working at the second, where you are placing yourself as the child to his adult. He may be your boss, but as a person you are his equal. Remember that, and it will help you to be a bit more forceful when you need to be, and feel less guilt when things go wrong. It will be easy for him to subconsciously place you as the child, due to your age difference, but you must resist that, whilst still retaining your respect for him as your boss, and for the experience he has. Just being aware of the dynamic can sometimes help, as you can make the conscious decision not to drop into the child role. If you can achieve that, it may cut down on the stories, as he may be slipping into the Parent role. Taking a more active part in the discussion will help prevent that.
Also remember, that whilst many of his stories may involve outdated technology, the thought processes and strategies involved with fixing problems is often independent of the technology involved. This may help you if you ask the question “So how do we relate that to ” to get things back on track. It is also true that things go in circles. Today’s Cloud Services model is not a million miles from Client-server, and even older Mainframe - dumb terminal topologies.
Please don’t think I am blaming you for your issues, it is just that if you want things to change, it is you that is going to have to change them. I don’t think that suggesting a 68 year old who is two years from retiring changes the way they do things will work!