Octomom's ex says he's the father of her eldest

Nadya Suleman

's ex-boyfriend says he believes he is the father of the Octomom's eldest son, 7-year-old Elijah. And he wants to prove it.

"I want to get a DNA test on that first kid,” says

Luis Ceballos

, 32, who served as Suleman's driver during the year she spent working as a stripper at bachelor parties, private events and a San Diego club.

Ceballos claims he and Suleman, 33, had unprotected, uh, relations "in the back of my limo. Right around the time she stopped stripping and disappeared, I found out she was pregnant. That is why I think the kid is mine.”

Suleman has insisted that her 14 children were all fathered by the same unidentified donor.