Thanks to all. No will that I know of. I jut want to know about his business.
If he had no will, then you'd be the sole heir to the estate. The sister would not be entitled to anything.
Left it how? There are two ways that the partner might be entitled to your son's interest in the partnership: (1) if that's what the partnership agreement says; or (2) if your son had a will that said that. Either way, if this is what the partner is claiming, he ought to be able to point you to the relevant language in the partnership agreement or the will.
You are. "OP" stands for original poster (or original post).
Why is the medical examiner involved, and why is it taking 3+ months? Did your son die under suspicious circumstances?
Regardless of what you learn about his business, he may have personal assets (cars, home, personal property, bank accounts, investment accounts, etc) to be distributed according to his will. If no will, under intestacy which that you and his sister may share in those assets. You'll still have to address the probate angle.
Then you've spoken to the partner. To what extent? Did you ask for documentation of the partner's claim? What did the partner say about that? If you haven't asked yet, it's something you'll need to address in detail.