Also had stock accounts for grandchildren with him as custodian and the kids names.
I assume this means that your grandfather owned one or more brokerage accounts and that each of those accounts designated one or more of his grandchildren as beneficiary. Correct?
The trust is straight forward but because he is personally the custodian (not the trust)
Just FYI, a trust has a settlor/trustor/grantor (the person who created the trust), a trustee (the person who manages the trust's assets) and a beneficiary (the person for whose benefit the trust exists). I assume you meant that your grandfather was the trustee. Obviously, he no longer is.
Will we have to go to probate?
I'm not sure who "we" are.
You don't have any obligation to do anything. If you are a designated beneficiary on one or more of your grandfather's brokerage accounts, then, in order to claim ownership of the account(s), you'll need to provide the broker with your grandfather's death certificate and probably fill out some forms.
Whether it will be necessary to probate your grandfather's estate is impossible to know based on the information you provided. I'm not sure why you ignored most of "adjusterjack's" questions, but answers to the following questions can help determine whether or not probate is necessary:
1. Did your grandfather have a will? If so, what does it say about who gets what? A "pour over will" is a will that leaves one's entire estate to a trust.
2. Was your grandfather married at the time of his death? If so, to what extent is his widow connected to the trust?
3. Obviously, one of your grandfather's children is your mother or father. Is your parent who is your grandfather's child still alive?
4. You mentioned "grandchildren" (plural). Are the other grandchildren your siblings, your cousins, or some of both? If any are cousins, are their parents who are children of your grandfather still alive?
5. What is the approximate value of your grandfather's assets,
excluding assets that fall into one of the following categories: (a) are jointly owned with another person with the right of survivorship; (b) are owned through a trust; or (c) are held in an account that designates a pay-on-death beneficiary?
Whoever is now the trustee of your grandfather's trust would be well-advised to consult with a local probate/trust attorney.