Is it normal for only one testicle to descend but not the other?

No. Normally, both testicles descend into the scrotum before a baby boy is born. Sometimes only one descends, and doctors have to move the other one down into the scrotum. Occasionally, both testicles do not descend and have to be brought down.
No. Both testicles should descend into the scrotum. If the boy has an empty scrotum then this needs to be pointed out and discussed with his doctor. Testicle that never descend can lead to future fertility problem and increase the risk of testicular cancer.
Needs surgery. The testicle needs to be located by ultrasound. Timing for surgery is usually between 6-9 months.