I don't post very often, but your post struck a chord with me. There are others who are far more qualified to offer advice and support on Dad's diagnosis, such as Lynn who has huge experience, but from an emotional point of view I do understand what you are going through and wanted to reassure you that time will help and things will calm down. My husband was diagnosed with an aggressive pca last August, just as I was coming to the end of treatment for breast cancer, so I have experience both as the cancer sufferer and the support to someone who has been diagnosed. Emotionally I was a complete mess for almost six months after my diagnosis, like you having been a single parent for many years and having only been married to my husband for a year, just when life was going well it felt like things were falling apart.
I found huge support from forums such as this one, but also the local MacMillan cancer support centre. They were amazing and I got councelling and support which has been invaluable in getting me through a horrendous 8 months.
As you say you need to be strong for your Dad and your little boy, but don't underestimate the toll this will have on you. You need to take care of yourself and if you can get some support. Do you have a local Maggie's or MacMillan centre? They exist not just for patients, but for friends and carers too and will help and reassure you through this difficult time.
Take care and look after yourself.