Your father shouldn't drink any fluids for two hours before he goes to bed. Additionally, caffeine -- which is in cola, unless it's caffeine-free -- can make incontinence worse. Alcohol
is also a diuretic (meaning it increases the production of urine), so drinking beer will make the problem worse.
Your father should ideally have his incontinence evaluated by a urologist or a geriatrician. The two most common types of incontinence are urge incontinence and stress incontinence. In urge incontinence, there's the urge to urinate and an inability to control the bladder. In stress incontinence, increased pressure in the abdomen -- due to sudden movements, laughing, sneezing, and coughing, for example -- cause urine to leak.
Because your father has Parkinson's disease (a movement disorder), it sounds like his incontinence is due to immobility -- meaning he can't make it to the bathroom in time -- rather than a bladder problem.
If your father is drinking a lot of fluids (especially diuretic fluids) just before bed, he'll likely wake up in the middle of the night and need to use the bathroom. If this is the case, you might want to consider getting him a bedside commode if he doesn't already have one. In the later stages of Parkinson's, a loss of inhibition regarding relieving oneself is also common.
You should take a look at the medicines your father is taking as well. Is he using medications advertised directly to the consumer -- such as Flomax or Detrol -- that might be interacting with his Parkinson's medications?
In today's world, people are bombarded with advertisements saying "Here's a way to fix your bladder problem." But consumers need to understand that drugs interact with each other. Certain people shouldn't take certain drugs. Again, talk to your father's physician, and be very specific in asking about drug interactions with Parkinson's medications.