Oh, Granny23, my dear I'm so sorry. But just remember that it's not intentional - he can't help it and all you can do is support him, love him and talk with him about both current and past, depending on which time frame he's in at the time - I love the "memories book idea"
With me it was my mum. She sent my son a cheque with no payment details - our address instead. DDs talked her into POA so they could shove her into a home as far from her friends as they could find, tried to stop us seeing her. Her neighbours kindly told us where to find her, so we visited that afternoon. She was thrilled to see us (me, GS andMIL, one of her best friends), but had to ask if I was her daughter. We were able to clear up a lot of loose ends in our relationship, which was always prickly. DDs quickly moved her and told her new home that she didn't want to see us, so shortly before she died, when we drove the 250 miles ew to see her, we were refused access. She always said she wanted her mum and to go home to Aberdeen, which she'd left 55 years earlier. Never saw her again, but thank goodness we had cleared the air before it was too late!
Our thoughts are with you. Hugs and love.