About the ages, I agree that there is more than one possible explanation and we cannot know absolutely for sure. However, as I read the text, I see no reason to think a year is anything other than a year. My thought on this is that God created the first people who were in his image more genetically “perfect” than we are. Humans, like all animals, are genetically determined to only live for a certain maximum length of time. Relatively small genetic changes would almost certainly allow us to live much longer. I speculate (and please remember that this is speculation) that the early human beings underwent some sort of mutation or mutations which gradually reduced their life span. If I am right, then probably this happened at the population bottleneck which happened during the time of Noah. This longer life span allowed people to populate the earth fairly rapidly.
About Sarah being 80, this was at a time when we know that human life spans had already fallen to more or less their current maximum value. Her having children at such an advanced age is clearly described as a miracle. And a miracle it was. There have been cases of women naturally giving birth as late as their upper 50s, but Sarah having Isaac at such an age was clearly a miracle, which would explain why Abraham and Sara really struggled to believe it would happen. This is the same Abraham who had such great faith, he was willing to sacrifice his promised son. Yet, he struggled to believe Sarah would give birth. That is how miraculous the birth was.
As for something to share with him, I suggest my book Reasons for Belief. This is the best book I have seen so far to cover most or all of the basics as far as reasons for putting faith in the scriptures.