@Ro***
, if you and your child's mother are in agreement it's a fairly straight forward but time consuming process at the family court. Depending on where you live I'd have you and the child's mother go to the family court and see how to get the process started.
Some family courts up-country MAKE you get an attorney but in bangkok you can do it without one with just the two of you.
This is an old but informative post on the Thai Visa forum by one of the admin's
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7-how-to-gain-parental-rights-as-a-father/