You say hes drinking and eating well, but he has to make sure that hes def eating enough, and to consume more energy dense foods... Tell him to drink at least 250 - 500ml water several times a day to keep BP above 100/60

If nothing works, then it is certainly something to get looked at more... I don't personally suffer any problems from low bp, but I also don't do a lot of exercise either.

http://www.water.org.uk/home/water-.../blood-pressure