Its may things that are contributing to this in my opinion, and no i am not a scientist with any sources but i things i have read over the past decade:
-industrial age with the birth of chemistry and organic chemistry leading to pollution, plastics with bisphenol A that has disruption in testosterone production and spermatogenesis
-plastics in clothing also affects testosterone production and spermatogenesis
-artificial fillers and preservatives in western diet. Low nutrition in especially key micronutrients that lead to spermatogenesis and testosterone production: zinc, magnesium, boron, etc
-low nutrition and high pollution in third world countries
-there have also been higher rates of male infant mortality and failure of testicle distention in western countries
-men, and young men are pretty much majority of the casualties in war, work and car accidents, homicides, suicides, crimes (and stress that comes from it) etc.
-more boys, and young men coming from single mothers with no positive male role model and fatherly figure.
-elementary, middle, and high school teachers predominantly female
-devaluing, alienating, demonizing, abandoning, mislabeling any inherit masculine qualities. Its labeled 'mansplaining' or 'toxic masculinity' or to check his 'male privelage'
the one thing i dont agree with is that low sperm count is related to kids playing video games and lack of physical activity. You can still play video games, watch movies, and do physical activity and eat healthy foods. Todays kids in my personal opinion have highly structured time slots of play dates. Having any type of structured activity for kids is a failure. Kids like 'flow' and 'spontaneity' especially boys, but unfortunately society treats them like defective girls.
Just my 2 cents.