Answer:
Most likely he has injured his Calcaneal tendon and the bulge is triceps surae or calf muscle.
Explanation:
The calcaneal tendon is the thickest tendon, which is present at the back of the lower leg and begins at the middle of the calf. It is covered with skin and fascia and connects the bones with the muscles.
The calf muscles or the triceps surae, are the muscles present in the calf of the leg.The calf muscles act via the tendon and cause flexion of the knee and the ankle.
While exercising, an overuse of the calcaneal tendon, can result in pulling of the calf muscles from the Achilles tendon. Thus causing a tear in the calf muscle, which appears like a bulge.