Oh, Rhiannon, poor you (all of you), I am sorry.
Was your DS close to your Dad? Had he seen much of him recently? Is your Dad awake enough to know he's there, rally and say goodbye?
Agree with crystaltips that if you do take him to say goodbye you should try to prepare him. But I think that if you don't take him, and he was close to your dad, both you and he may regret the loss of the last chance to see each other.
Sorry - don't know enough to really help. You know them both best - what do you think would be best for them and for you? Is your Dad alone now? You don't mention your mum.
Don't forget that even when people are near the end they often can still hear and know who is there. If you can take DS tomorrow I think it would be better for all of you if you can - yes, your DS will probably be upset but it will help him understand about death, and you being upset, and it won't be keeping anything from him.
Good luck. Hope he passes peacefully.