My 2 years boy having only one testis by birth. is there any problem to his sex life and bad impact on body immune?

No: He should have normal life and health as anyone else, but make sure the other testicle isn't just "undescended", meaning it is not present but hiding up in in his pelvis. Please let a Pediatrician/Pediatric Surgeon examine your child thoroughly, wish you both wellness"

Needs evaluation: If one testis is hiding in the abdomen, it has a higher risk of developing cancer in his lifetime.There is also a suggestion it could impair the success of the normal testis.Have him evaluated. The "absent" testis needs to be brought down to normal position or removed if present. After this is done, his life should be fine. It only takes one good testis to be fertile.