Hi Rachels103 and welcome to MN.
Its going to be a very hard time for everyone..you especially. I am a firm believer in the saying 'time is a healer'.
When I lost my lovely Dad, almost 3yrs ago (to a drunk driver) it was the most awfully sad time and I really didn't know how I was going to cope. I was very close to my Dad. He wasn't only 'Dad' he was my best friend. My kids were 18mths and 7yrs at the time.
I pushed everyone away. I didn't want anyone in my life if I couldn't have my Dad. But if it hadn't have been for my kids I would have gone rapidly down hill - they kept me going. My Dad had a lovely relationship with my eldest. Even now I miss talking to him about how she is doing at school etc. He took such a huge interest in her academically. I don't have anyone to talk to about her now. My Mum really isn't interested and we have no contact, whatsoever, with DH's family.
It will be tough for you and your family for a long time. You clearly loved your Dad enormously. He is looking down on you right now. He will be so proud of you and what you are doing bringing up his lovely Grandson.
Whenever I feel down and miserable, and yes I've been there so many times, I just think to myself that Dad wouldn't want me being like this and to try and keep myself upbeat about everything. He was never miserable and always had a smile on his face.
Talk to your DS about 'Granden'. Point to the stars at night. Encourage him to give Granden's star a wave. My youngest, who is almost 5yrs old, always waves to her Grandad before she goes to bed. She says goodnight to him and blows him a kiss. Whilst she never really knew my Dad because she was so young when he died I do talk to her about him and show her photos etc and I tell her why he is 'up there' and she comes with me to his grave. It's important to include your kids I think because it helps them to understand.
It is hard, I know, to know that your Dad won't see his Grandchildren grow up and especially as he was such an important person in your life and your DS's but just take one day at a time. It does get better. You will go through dips and troughs. I still cry now especially on birthdays and at Christmas. My youngest started school last week - I know my Dad would've rung to see how she got on. Obviously I missed that phone call. But I know he's watching and that is a comfort to me.
Take care.xxxx