I would not want him drinking his protein just supplementing it in between meals.How tall are you? IIRC you are in your 20s and I had trouble gaining during that decade of my life as well. Weights are your best bet along with forced feedings. Remember: It's easier to drink 1000 calories(protein shake) than it is to eat it.
True. But I think a good two or three day a week full body routine with extra calories would be a good beginning.
Three solid meals or more per day every day plus three smaller meals everyday. Depending on his appetite this could be harder than the actual training.
Depends where he is physically.
I personally do not like full body routines but they work for some to a point.
I would start him with the very basics of weight training. Lots of compound movements and mid section training every training day to reduce back injuries.
The mid section ie the area around the belly button is the weakest part of the human body. It must be strengthened.