Did you mean that he stops breathing 40 times every hour? If he has obstructive sleep apnea, and stops breathing 40 times every hour, it's in the moderate to severe range. Untreated, it can aggravate or lead to a variety of medical conditions such as high blood pressure, depression, obesity, heart disease, heart attack and stroke.
Unfortunately, this condition is a structural problem and needs to be dealt with in a physical manner. To begin with, a good diet, regular exercise and weight loss is the first step. Try not to eat or drink alcohol within 3 hours of bedtime. Not sleeping on his back can also help, but it depends on if his sleep study showed that it made any difference.
The two other ways of treating obstructive sleep apnea is either a dental device that pulls the lower jaw forward at night (pulling the tongue forward), or surgery, which is a last resort. All these options work to various degrees, depending on your father's condition and the expertise and skill of the treating doctor.
There have been studies looking at using certain antidepressants and even acupuncture in the past. The two studies that I recall lowered the numbers on average by about 50%!
But even if if his number dropped by 50%, he would still stop breathing 20 times every hour.
I recommend that he follow-up with his sleep doctor and give CPAP a try. Work with the sleep doctors and the equipment company to find a mask and device that works for him. I also recommend that he join and take part in support groups such as ones offered by the American Sleep Apnea Association (in person or on-line).
Hope this helps.
Thank you a lot, that helped a lot! And yes, I meant stops breathing 40 times.