Worried Mum

Something has obviously raised some doubt in his mind , I would guess, about his dad and in his mind has been blown out of proportion. Has he been having any medical checks recently, such as a Wellman check that many GP surgeries offer, that has involved blood tests and has been given incorrect info as a result?
Or maybe he has always had a chip on his shoulder since childhood, and wrongly believed that his father didn't care for him ?
Either way, I don't think you can be subtle about it and need to ask him straight out why he thinks this way. When he would have been little, photography and developing films was relatively expensive and I'd guess a lot of people in their 50s will be in the same position with the lack of photos

Has there been some specific event that kicked this off, loss of job or relationship breakdown? He may have come to a point where he is reassessing life. I think I would wait for him to bring it up again and when he does, this time, try and find out what the problem is. Compared to the vast number of photos taken today maybe it is difficult for the 40/50 age group to appreciate why there may be so few pictures of them.

Does he have a partner who you might discreetly talk to in a general way to see if he is upset about anything that you can deal with? Is he having a crisis at work, or is there some upset in the family, like ill health of someone else and it is making him feel that he needs to keep picking over the past?

oh, so sad. I hate fall outs especially with family.

if that were either of my two sons I would meet up and ask,
but then all families are different aren`t they? depends what your relationship was/is like with him..

I would meet my sons

Is he having counselling at all? Sometimes this throws up ideas by dredging up the past.