Loosing someone or even something changes your own life balance, our place in the world. particulay when its a close family member or parent and you suddenly realise your own mortality. My parents both died when I was in my 30's and struggling with family and marrage and it was very hard, I would start to cry when I saw someone in the street or a film or music, still have a lump on occasions and its years ago now.
Talking and going on a memory trip is good, get him to talk about past fun times with his dad and if you can, get him to have a smile, I found doing some voluntary work with people in need helped put my problems into prospective, one thing I hated after my mum died was going on holiday so soon after her death, I needed my surroundings to stay familiar in order to deal and adjust to my new situation!! It will pass but like any hurt, you end up with a scar.
While writing this I cannot help remember a horrid moment in my working life when I got some one to talk!!!!
I used to supervise home/family visits for children in care, this time it was a girl of 16 with learning difficulties who had been removed from her home as a toddler due to suspected abuse, she had no family contact and her gran was ill so it was my job to take her for a visit for the first time in years and it was 3 days before Christmas.
After introductions there was no conversation so I looked for conversation starters, for ex:-
An add on the TV with dogs, ME, I love dogs do you?
GRAN, just had to have mine put down after 12 years

SLIP UP 1 Then, after a while, ME, that Christmas card is pretty,
GRAN, its from my eldest daughter in AUstrailia and its last years as she just died of Cancer

SLIP UP 2 After a while, have you other children (I saw a family photo)
GRAN, I had three, they are all dead now, the eldest jumped off a multi story car park 6 years ago and the middle daughter set fire to herself in the middle of town!
BIG BIG SLIP UP 3 So sorry!!!
I must have left her so upset and I wish I had been told before I went what a sad life this women had!!!!