What are the visa options for a foreign father of a Thai child without legal parental rights in Thailand?

While you're doing the family court process you could cancel the volunteer visa extension you're on with documents from the foundation at the immigration office that issued it (THIS IS CRITICAL & HAS TO BE DONE)

Then you could go to the thai consulate in Savannakhet Lao and get a 90 day single entry Non-O visa based on being the father of a thai child, you don't need the parental rights decree just the birth certificate, the mother's thai i/d and her house book listing for 2000baht

That would give you a 90 day stamp when you entered, and you could get one 60 day visit thai family extension on that giving you almost 5 months in country.

Hopefully that would be long enough for the family court case to conclude and then with the parental rights decree and 400K baht in a thai bank account you could get a year extension based on having a thai child. .

As you were not married to the thai mother, you have no parental rights to use the child as a reason to get a long stay visa/extension. You have to go thru family court to get granted legal parental rights. That process takes about 6 months (mostly waiting) on an uncontested parental rights case (where the mother is okay for you to get parental rights)

GO to the family court in your area with the child's mother and see how the process is done. Believe me you're far from the first foreigner to have to go thru thus process to be able to use the child to get a long term visa/extension inside the country.

Until you do that, all you can get is a 60 day visit thai family extension and you can do that ONCE per entry to thailand.