DPs dad just died

[Hellmouth]

I don't know what to do, I feel at a loss because there is nothing I can do to help.

I feel so bad for DP, and for my son, who is only 5 months old. He will never know his grandfather now.

Sorry for your loss you're right there is nothing you can do except listen to your DP when he wants to talk about his dad you can also share some memories of his dad (if you knew him) which is something I found helpful.
My son will also never know his grandfather but I show him pictures & I'll tell him stories when he's older so I hope he'll still feel part of his life