Old hernia scar from 20 yrs ago showing a slight bulge. Goes away with a little pressure. Will this heal on its own? Does 29 yr old male need surgery?

Need check. Recurrent hernia is a known entity and repairs may be indicated.
Probably yes. If you are now 29, and had hernia surgery 20 years ago, that makes you about age 9. You did not specify where the hernia was located, but in an infant, some twenty years ago, I am doubtful that mest was included in the closure of the hernia repair. After 20 years, and a lot of growth, sounds like you hernia repair needs revision, perhaps now with the placement of mesh, to make if more permanent.