I think he just needs to relax. The methods you've listed that he's tried so far are all about adding more pressure and straining the muscles. This is only going to tighten all the muscles in the area, including the urethral sphincter.
If I'm having trouble peeing, I literally just on the toilet and try to relax as much as possible. Deep breathing/relaxation techniques will help. If he wants to take a book or magazine in there to read and try to zone out that might help too.