just won a 2-year legal battle against his baby mama -- with a judge just declaring he is the legal father of 4-year-old Gus ... TMZ has learned.
Jason had a baby with ex-GF Danielle Schreiber
, and after breaking up -- getting back together and breaking up again -- she fought hard against his bid to become the legal father, citing a quirky law.
According to the law ... if a man donates sperm to a woman to whom he is not married and a doctor facilitates the insemination ... the man has NO claim of custody or parental rights.
The trial judge ruled against Jason
, he appealed and the appellate court said the trial judge was off base and sent it back for another hearing.
The second time was a charm for Jason. We've learned the trial judge just issued a sealed 45-page opinion ... concluding Jason is the legal father.
The judge ordered Jason, Danielle and their lawyers to try and hash out a custody, visitation and support agreement ... but if they can't, the judge will make the decisions.
This case became a rallying cry for father's rights -- and an enormous victory for Jason.UPDATE
: Schreiber's rep tells TMZ, "The proposed statement of decision is not intended to be public nor is it final."