What Causes Restless Sleep With Vivid Nightmares?

Question: 61 yr old Husband has been experiencing recent behaviors that are becoming concerning. He has been waking suddenly after having dreams of snakes or other frightening things, or jumps up out of bed running from something. Most recent episode involved spiders. They are so vivid he remembers them. A couple of days ago, he forgot about my brother passing away and reacted as if he had heard it the first time. He later admitted he remembered it, but vaguely. He has had other memory issues over time that I am now starting to piece together. When he was in his late teens-early twenties, he used to box. He was hit so hard one time that he has a severe deviated septum that cannot be repaired. He talks about the many blows he took to the head, and he finally quit because of it. Over many years, I have thought he might have had some learning disabilities, as he cannot process some things very well, has been mildly depressed, sleeps a lot and has general lethargy. He has other health issues and takes meds, so I chalked it all up to this. But this recent memory issue and now waking in terror of something from his dreams makes me wonder if he isn't now emerging with something related to his boxing or if he is starting to develop dementia.