@Hollyhooha, im so sorry for your loss. it sounds like you and your son have some very special memories together. whilst you know in your heart that you couldnt have helped your son physically, you feel your presence, just being there with him might have helped.
perhaps reflecting on some of those special moments will bring comfort. connecting with your sons memories in a different way such as writing a letter or cooking his favourite meal. maybe donating or volunteering for a cause that might be dear to him. we are here to listen and support in ways that we can.