Any suggestions on what to talk about to my FIL who is 88 & showing signs of dementia?

Hi - my fiancee & myself have recently moved in with FIL who is 88 & showing signs of dementia or just aging in general. At times he is forgetful etc - we will start some neurolgy testing etc in the next few weeks. MIL passed away very unexpectedly so we moved in to determine what needs to be addressed etc. I feel most times I am struggling to have a conversation with him - he is sweet as can be but is very, very quiet - I will take any suggestions on how to strike up a conversation. He is still somewhat active & we include him in almost everything. We have just turned 50 & this is all new to us. thanks!