So, here's my question. Is he just a late bloomer, or is there something else going on here?"
I've heard of "late bloomers" but he should be showing signs of maturing by now. His balls should be getting larger; his cock, too. And at least small patches of pubic hair should be showing on either side at the base of his cock.
Lack of erections is 'nothing to sneeze about,' as the saying goes. And, at his age, he should be starting to tear through the cumrags like no other time in his life. I'd say it's definitely time to see his pediatrician.
One thing to consider between now and getting to the doctor: What kind of foods/milk have you been giving him? If the answer is Soy anything, that could be part of the problem. Soy is chock full of estrogen. It has been linked to the development of breasts in males, from pubescent teens to men in their 50s. It is also theorized to be a link to the increase in male breast cancer.
His physician should refer you to an Endocrinologist for further testing. Because that second wave of testosterone should have been washing over him by now. He should be experiencing boners galore, and wet dreams if he isn't jacking off. You need to get your son on the right set of rails here, or it could spell trouble down the road: The inability to produce swimmers, the inability to get decent boners, and the inability to cum.
And, that would be a damn shame.
Some of my Faithful Readers might be able to offer some wisdom.