Sending condolences.
A couple of points from your post.
1. You are still his next of kin on paper.
2. His son is 24.
24 is awfully young to undertake the responsibilities. I don't know what your relationship is to the young man but could you call him and ask for half an hour of his time to discuss what you believe to be important for the funeral of your ex partner? If he says he doesn't have the time put some thoughts on paper and give them to him - copies to other family members maybe.
I say "what you believe to be important for the funeral" because, in time, I think you'll find these things less important. When someone dies unexpectedly it can feel that no-one has really taken into account the wishes of the deceased. His musical tastes, what his wishes were.
When my darling Dad died at 80 I asked my Mum what his wishes were - "I don't know" was her startled response. If you know what your ex partners wishes were, then tell the son, but if he decides to go in another direction try and find it within yourself to shrug and let it go.