My ex-husband just passed with no will; do his assets go to his parents or his child?

Since your husband died without a Will, he is said to have died "intestate".  Accordingly the intestacy laws of the state that he legally resided in would prevail.  Typically, it would be to the surviving spouse, if any, and children (I will assume that he did not remarry prior to his death and had no other children).  Therefore, your son would be your ex-husband's legal heir.  As such he would be entitled to all assets that pass through probate. 

However, your husband's interest in the house may or may not be such an asset.  If title to the house was held as "joint tenants with right of survivorship" (JTWROS), then by operation of law his share of the house will pass directly to his parents.  If title was held as "tenants in-common", then his share becomes part of his probate estate and, as stated above, will be distributed to his heirs; in this case your son.

Note:  If the deed is silent as to how title was held, there is a judicial presumption that it is as tenants-in-common.

I am sorry for your loss.

Your ex-husband died Intestate and so the intestacy laws - those that list who is to receive the assets of an estate in order of priority - will govern.   It is my understanding of the California Intestacy laws that your son is the ONLY heir to your ex-husband's estate, if that is his only child, as parents are cut off when a child is born.  Your rights were terminated upon divorce.

However, that does not but off rights that are jointly held between your ex and his parents, such as owing a house together, joint accounts, etc. 

You need to seek legal help immediately with all this.  Someone has to make application to the Court to be appointed Personal Representative of an estate proceeding for your Husband's estate. If his assets are being taken improperly - dissipated - then the personal representative will have the power to make things right.  You may also have to go to Court to appointed as the legal guardian - you are the natural guardian - for your son in order to have the power to act on his behalf.  Good luck.