My soon to be 45 year old son is having major memory problems. Has anyone else gone through this at a young age?

PTSD is directly related to your son's memory loss:

Can PTSD cause memory problems?
But one of the most pervasive symptoms of PTSD is not directly related to emotions at all: individuals suffering from a stress-related disorder experience cognitive difficulties ranging from memory loss to an impaired ability to learn new things.

If you Google it, a ton of info will come up explaining how PTSD relates to memory loss.

I don't think people realize the scope or the magnitude of PTSD and how it affects every aspect of the sufferer's life. Please do not suggest your son discontinue any of his prescribed medications. Lack of sleep ALONE can and does contribute to tons of memory issues, never mind all the rest of what he's going through.

I had a case of PTSD in 2000 resulting from an emotional trauma, which, I'm sure, was nothing by comparison to what your son is going through. I will tell you this: it was THE most awful emotional/mental and physical experience of my life and affected me in ways that was mind-boggling. I finally went to the doctor and was prescribed an anti-anxiety/anti-depressant which helped me a lot; I was finally able to sleep and the startle reflex calmed down so I was able to stop wearing earplugs in the house, just to function day-to-day.

Wishing your son and your family the best of luck with a difficult situation. Sending you a hug and a prayer for a good resolution.

Maybe an odd suggestion, but an older, experienced hypnotherapist might be able to help him as well.

Think it would be great help to DIL if
you could be more hands on …
she’s probably very exhausted doing all the care … you could offer stay with your son and give her regular respite or pay for some help !
she's probably not telling you how hard it really is !!!

Does he use drugs or alcohol?

As a Coastie, he is eligible for VA care, isn't he?
Especially if the PTSD is related to events that occurred during his service years, the VA is responsible for helping him...and the VA has lots of resources and expertise in this area.

Get him enrolled in VA system if he is not already there.
Be sure he has signed a document to allow his wife to talk to all health care providers - a waiver of his HIPAA rights to allow his wife to receive the HIPAA protected information.
The information will be too overwhelming and frightening for him, so he won't really hear or retain the choices.
The VA has comprehensive care - meds, trauma trained therapists, group with other service members with PTSD, day programs, and short term inpatient stays. Not all services in one place, sadly.
But a good place to start.

Sounds like family members or friends who are comfortable being outside with him (can calm him if he starts to get nervous about something) helps a lot.
(Mancave and shadows and video screens may trigger some of the PTSD).

Also, I wonder if he meets the diagnostic criteria for TBI (traumatic brain injury)? A medical determination, but that can help find most appropriate treatment. Sometimes meds don't work so well and therapy, structured activities, etc.
Let us know how he is doing, and your DIL and you as well. Will be sending good thoughts.

I should add, I'm going to call my daughter in law's mother. She and I get along great and she'll let me know if I need to come.

Was your son in any rough sports in high school or college, like football or hockey? If he had any concussions, even one, it could impact his cognitive abilities earlier in life, like the CTE that some pro football players are posthumously being diagnosed with.

Of course this is taking a toll on everyone!. There is early onset Alzheimer's but from what you tell us about his health I would be more inclined to think otherwise. The tumor may be returning, meds may need to be adjusted. With these drastic changes, I would get him to his neurologist asap.

I'm so sorry for this distressing turn of events for your son and his family. I can't answer the medical portion of your question, but can only encourage you to maybe consider contacting your DIL and seeing if she can use your help at their home (like help managing home life for their kids) while she is at appointments with him and on the phone, etc. for an extended periods of time. If he's 45 then I'm assuming you are perhaps retired and maybe in a flexible position to help them. As a mom of 3 sons and grandkids, I wouldn't want to have any regrets that I didn't do more when I could have. I'm not trying to guilt you or judge you, but sometimes people (like your DIL) may not ask for that kind of badly needed help because they think it's too big of an ask, or she's just too stressed to think about the details of you coming, etc. Even if you're just running errands and cooking meals for them, it would be such a huge blessing. I'm sure there are more things you can do from a distance as well -- continue to make suggestions to her so she doesn't feel like she's asking you for favors. Wishing your son and his family much success in achieving better health and for an abundance of peace in their hearts.