My son plays basketball year-round. He complained of pain in the lower front portion of his left knee all last
year. He no longer has pain, but he has a hard bump where it used to hurt. Should I be concerned?

You are describing a condition called Osgood-Schlatter disease.
It is not really a disease, but rather an overuse syndrome related to the
growth process.
Just under the patella or kneecap is a short, wide tendon called the
patellar tendon, which attaches to the front of the tibia bone just below the joint line. When the knee is straightening, tension is placed on the patellar tendon attachment site.
As a youngster, this attachment site has an underlying growth plate.
Excessive running and jumping during the adolescent growth phase can irritate and stimulate this growth plate.
This stimulation can produce more bone growth resulting in a pointed
prominence of bone.
The good news is that these symptoms usually disappear by age 17 in boys and age 16 in girls, but the bony prominence persists into adult life.
Osgood-Schlatter disease does not affect the function and stability of the
knee. But if the resulting bony prominence in front of the knee is large,
prolonged kneeling may be uncomfortable.