His knee does appear swollen, but I doubt whatever caused the apparent swelling happened that day on the beach. If Aleksei had hurt himself badly enough to make his knee immediately swell up, it's probably safe to assume the IF wouldn't have stayed on the beach long enough to snap photos. An injury like that would have been painful and dangerous, and likely would have required immediate medical attention.
I think it's much more likely that Aleksei was *recovering* from an attack of hemophilia when this picture was taken.