I'm assuming you're still with me and haven't either fainted at my use of the word penis or died from laughing or embarrassment.
JP, a fellow Lab breeder from the UK, put it best when she said in explanation:
"The dog is one of the rare animals to possess a penile bone, called an os penis. It is this bone that produces the partial erection required for penetration during any mating. Real erection is stimulated as entry to the vagina pushes the penile sheath back and full erection is completed only after penetration has taken place. So, that little bone plays a very important role in reproduction.
In some breeds, it can even be the source of problems. As the os penis surrounds the urethra for part of its length, the diameter of that tube narrows as it passes through the os. Stones can get lodged in this region. In some breeds, the os penis is often the anatomic site of the dreaded urinary obstruction - a large stone dammed up where the urethra narrows. As it cannot find an exit, the urine starts to back up. Urine reflux can quickly lead to death. It is thought that the growth of this bone in youngsters is regulated by testosterone and reaches maturity at around fifty (50) weeks. In breeds prone to urinary stones, early castration should be avoided to allow the bone to grow to its full size."
Okay, cover your eyes if you're modest.
You'll notice the head of the prepuce there on the left and his testicles on the far right.
So the next time your boy swells, I hope this article will help you realize it's not broken. It's just him being a normal male.