His Dad doesn't always turn up for fortnightly visits

We have regular arrangements for my son to see his Dad. He really looks forward to Saturdays and sometimes his Dad just doesn’t show up. I can’t bear how sad this makes my son.
This is a difficult one; after all you cannot force your son’s dad to have contact. You could write a letter to him explaining (in a non-aggressive way) that your son gets very upset when he doesn’t come as arranged and ask him to reassure your son that he still loves him and to stick to the arrangements. It is tempting for you to criticise the other parent to your son (“Typical! Your dad never keeps his word” etc) but try not to do this. Instead tell your son that his dad loves him and will come if possible but that sometimes he can’t get there. Have a couple of treats in mind, which you could both enjoy if you get that extra day together because dad doesn’t show up.