First off, true BXO would be extremely rare in a child of your son's age. I wasn't implying that that was something he may have had; I was merely saying that some adult men who have had that then experience preputial scarring. Please don't jump to any conclusions.
Now, let me put your mind a bit at ease about scar tissue -- which again, is unlikely the culprit here. (Remember Occam's Razor: the simplest or most common explanation is usually correct.) Actual scar tissue in that region would present as a slightly discolored (lighter) ring -- even when you're not pulling on it -- with redness around it. It sounds like what you're seeing is nothing more than blood leaving the area when the skin is stretched tight. Perfectly normal. Maybe stop doing it.
Scar tissue forms in response to trauma or infection. It is not clear that your son has had either to the obvious extent that it would leave such damage. Moreover, as scar tissue forms, a key characteristic is that collagen is up, yet fails to array itself in its normal logical pattern for a period of 3-6 months. After this time, it then starts to rebuild in its proper orderly array. This process is called "remodeling".
Even if your son did have an egregious infection 2 weeks ago (which sounds doubtful), there would be no way to tell the extent of any possible scarring before Christmas. This is because toward the end of the process, the tissue regains some of its elasticity from the successful arraying of collagen.
I don't think there is a basis for saying any of this is going on. It sounds like your son is a normal boy with a later-developing foreskin. I urge you to just ride things out quietly until he's on the other side of puberty. By then, he will be able to research for himself how his development is coming along, and what he can do if there's something to address.