testis came out from scrotum

My 6 years old son report me sometimes his left testis came out from his scrotum up to abdomen and came-down again automatically or through pressure. My doctor told me its  needs no treatment, my question is is it really quite normal? Dose he suffer In his adult age? Is this problem lead to any problem of infertility?

[uther]

Im 37 and my left testicle does and has been able to go up inside my abdomen ever since I can remember.  Im not a doctor but I was told by my doctor at a young age not to worry about the condition.  The testis are originally in the abdomen and descend intot the scrotum. It only causes a problem is there is a completley undescended testicle. Then it has to be brought down into the scrotum. The testicle otherwise can be succeptable to testicular cancer.  The area it retreats through is the inguanal canal i believe. If it is a chronic condition, I think a doctor has to stitch it to the inside of the scrotum to keep it from retreating all the time.  Hope this helps. Just my opinion.