Coached a number of OS kids. Often they run like they have rigor mortis. No much to do but wait it out. Second hand message I've gotten from the doctors (via the parents) is its a tolerability thing. You can run as much as you can tolerate the discomfort. For some, its not much, for others its a minor aggravation. But seek actual first-hand med advice.
Its also indeed not uncommon even for boys, to plateau or even regress during a rapid growth phase. For ladies, its a near lock, but it happens to boys too.
That is a perfect description. His form looks like he's in rigor mortis. Almost looks like an old man running, especially the slower he goes. He also seems to bounce up and down instead of projecting movement forward.
For those arguing in this post, yes, he does have some knee pain.
I'm no doctor but the way I interpret Osgood-Schlatters is that the leg bones grow faster than the surrounding muscles, ligaments, tendons, etc. I'm assuming a sudden growth spurt would be a prime indication of this condition.
And a bump on the tibial tuberosity.