Did Wesker know he had a son, or did he just not care? If he had crossed paths with Jake before RE5, would that have affected his decisions that ultimately led to his demise? Would he try to sway Jake over to the "dark side" so to speak or would he simply try to kill him? I think he would be impressed with his son's combat abilities at least.
I think more than anything, he would find it amusing that his son and Birkin's daughter are working side by side. On that note, I wonder what William would think about Sherry fighting B.O.W.'s that he would probably view as remarkable? Sweet irony.