Sorry for the delay my friend-
I was called to the hospital and did not return until a short while ago-
The testicles MOST often shrink from the use of injectable male steroids (testosterone) as the body's normal composition is regulated by the amount of LH, or leutinizing hormone. If too much testosterone type steroid is absorbed by the body, the body's feedback loop to keep the testicles making testosterone is shut down. When this happens, they shrink.
Other medical issues which can do this are alcoholism, low thyroid conditions, decreased blood flow conditions (arteriosclerosis), varicocele ( clump of abnormally working veins), and trauma even from hernia surgery. Also, liver disease from other causes can result in atrophy (shrinkage).
Alcohol use would be the next most common cause of drug related testicular atrophy followed by tylenol chronic use, marijuana chronic use, chronic opiate (vicodin/percoset type pain relievers & MDMA use (ecstasy).
It is possible for him to be abusing illegal drugs, but Anabolic steroids and Alcohol blow the others away in terms of frequency. I don't think the shakes are to blame, but men who take muscle mass builders often use anabolic steroids. Just food for thought. If there is something (a drug) you'd like to ask more about, let me know and I can hopefully dig up more information.
Feel free to ask any question you need.
I hope this was helpful.