Well, a couple of things.

It is hard to tell, as this is not a great video, but the elongated, spindly nature of his limbs suggests a marfanoid habitus. This does not mean marfan's syndrome, specifically, just that his features follow that pattern. This also does not appear to be due to simply slow growth plate fusion, as would be the case with several disorders involved with eunuchoid appearance (long, spindly limbs again) due to slow maturation.

So, the top three on my differential would be:

Marfan's Syndrome. As explained above, he has a marfanoid habitus, and there is a reason other syndromes which produce a similar appearance are described as presenting with a marfanoid habitus - it is simply more common than most of the others. However, he has grown quickly, which does raise the question of whether or not Marfan's alone could explain it. As such, there are several other possibilities.

Given the massive height difference, I would say pituitary gigantism is likely as well, meaning a pituitary tumor is playing a role. The fact that he is an adolescent would fit the mixed presentation - he does not really have most of the features associated with acromegaly because his growth plates have not fused yet. However, he seems a bit too spindly to be standard gigantism (which tends to make people big but also not quite so thin), though it is still possible.

Another possibility is untreated Homocystinuria. It would fit the physical presentation as seen in the video fairly well, but I don't know if he has any eye abnormalities so it is impossible to narrow it down.

For Marfan's: Aortic dissection is the most common cause of death. If untreated, it tends to kill them young (32 was the average before we had good treatment options.). Isaiah Austin was given an honorary NBA draft pick after he was diagnosed with Marfan's. The increased danger of aortic dissection made it dangerous for him to play professional basketball.

For gigantism, heart failure is common due to enlargement of the heart associated with having to pump so much (most will have high blood pressure). Type 2 diabetes is also common.

For Homocystinuria, blood clots are the issue, with all the things associated with them (stroke, etc).

I could be wrong, of course. But he very clearly has a Marfanoid habitus, so something is going on there.