Depression vs Dementia in Parent?

A little background. I am in the US. My Father is a 66 year old widower. My mother passed from Pancreatic Cancer in 2014. He has 3 adults sons: Me(31) and my brothers are 36 and 39. He was/ is a very successful Attorney with his own long standing practice. But outside of work my Mother ran his life (in a good way). They met at 18 and he had awful parents (not abusive. Always present. Just awful as parents), so her family was his guide for what a proper family looks like.

Anyway, after her passing he found a good social Group of people who have lost spouses. It is great for him. He started dating a woman who was in the group who was very sweet. They dated for almost 2 years but it ended back in April.

Ever since then he has shown clear signs of depression ( he's not eating, and this is a man who LOVES food) he's withdrawn and disinterested in even seeing his grandkids. His prior psychiatrist passed away, so we had him see both a psychiatrist to adjust his meds., as well as a psychologist.

I'm obviously concerned about that, but that's something that can get better. What REALLY has concerned me lately is increasing memory loss (short and long term).

1. He and my brother are the two remaining partners in their law firm, the others have passed. (im also a lawyer but I live 1000 miles away) my brother says he can't let my Dad do anything anymore. Things my Dad has known how to do for years, he gets nervous about and can't do it. Simple things according to my brother. And this has spanned the entire time since my mom passed. Even the 18 months or so he was very happy w his GF.

2. My Other brother's family was all sick this week. Himself, 1 year old son and wife. Brother asked my dad to pick up stuff at the grocery and he said "I don't know what the stuff you like looks like"

3. My wife and had an offer accepted on a house the other day. In a group text I told my Dad and both of my brothers that I thought it was a good Omen that right after we put in the offer, a Carole King song was playing on the radio( Carole king was my moms FAVORITE. She picked Carole king songs for wedding first dances etc. everyone knows how much she loves her). My Dad did not get the reference at all ( and it's not something he would miss if he were mentally healthy. My brothers and I were pretty taken aback)

4. So that conversation was Saturday. Our offer was accepted and I sent them all a group Text again and he said "congratulations!" Fast forward yesterday, my brother tells him he should be interested in my house. So he asks to see pictures. Then he says "when will you know if they accepted the offer?" And I felt like someone shot me. I said "I told you yesterday they accepted" and he didn't react like he was just spaced for a second. He went back and looked at our non-group text and said "my last text to you was to let me know when you heard" and I then had to explain to him that it was not in that thread, the group text that I told him it was accepted and that And he said congratulations to me.

Obviously no one here can diagnose him. But I'm wondering about the interplay between Depression and Dementia. I know depression messes with your entire brain chemistry, and memory is effected. Am I putting the cart before the horse in worrying this may be more than just a symptom of Depression?

I am visiting my family this weekend and i have talked to him about seeing a neurologist to give me piece of mind. Any other thoughts?