It is common for older men to have to urinate more frequently than younger men.  I have the night problem a bit also, but don't have difficulty during the day.  About 20 years ago I had a TURP which is also known as the roto-router job for men which clears and enlarges the urethra where it goes through the prostate.  That is a common procedure for an enlarged prostate and is hell for a couple of days but heals quickly.  That resolved my difficulties for several years until they returned as described earlier a couple of years ago.  Because I'd taken saw palmetto for a long time without good results, I tried Flomax for about a month but it has a side effect of inhibiting orgasm, not a good side effect.  Then in one of one of Dr. William Campbell Douglass' newsletters I read that lycopene is much more effective than the saw palmetto and I tried it, (after quitting Flomax) and found a significant improvement.  

Why do I just have difficulties at night?  Have been told it is probably just a matter of how the bladder lies within the body at that time and which nerve or nerves it is prodding.  Because of your dad's weight I would guess that there's pressure on his bladder while he sleeps.  A few years ago I was about 40 pounds overweight myself and from experience I know that added weight means added water retention, which brings up another point.  By eliminating all liquids by 7 pm (if you go to bed at 10) you cut down your night trips.  I find it even better if I stop by 6 pm.  I no longer have a real problem though I do get up once or twice a night but any urologist is going to tell you that's pretty much normal at my age - 78.  Yet I attribute my improved nighttime bladder control to lycopene and would suggest that for your dad.  Think that it was only a week or two before I had good results.