Juvenile -- I'm Not So Sure My Son Is MY SON

Juvenile is locked in a child support battle with his baby mama over his 13-year-old son ... but now Juvenile has learned the boy might not be his and he wants him to take a paternity test, this according to new court documents.

As TMZ first reported, an arrest warrant was issued for the rapper in February when he fell behind on child support to the tune of $160,000. Juvenile told TMZ himself that he's a good father and that his son stays with him most of the time.

But now Juvenile has fired back in a big way ... filing court docs in Louisiana saying he never established paternity when his son was born and didn't even sign the birth certificate at the time.

Juvenile claims he only recently learned the boy's mother was having sexual relations with "at least one other man" at the time and that he was "fraudulently induced" into various child support agreements.

He is requesting a paternity test and if it turns out he's not the father ... he wants all judgments against him released AND he wants back every dime he laid out for the boy for the last 13 years. Ouch.

The two sides are set to be in court next month.