My bladder is a "new" army surplus "skilcraft" MOLLE II 100 oz bladder and imparts no taste whatsoever. Instructions advise baking soda in water to freshen and a small amount of bleach in water for more serious cleaning. I bet your son's hydration bladder sat with water in it for too long at one time, or some such thing. I am a non smoker and there's no after taste from the bladder I use now. You do have to drain the water that is sitting in the tube as you hike or walk, as it gets warm, but the water in the bladder stays amazingly cool. ( high 80s here and the bladder kept the water relatively cool in it's original carrier and in the main compartment of my pack.
I really like the 100 oz bladder and bite valve. Oh, and to mention it, it's not a matter of taking my time, it's just easier. I have to take my time. I had to stop for a rest to catch my breath about every 50 feet for about the first half a mile or more on this last hike! shocked eek
Once I got going (although a smooth trail,it's steep, especially at first) I did better of course. So, it's not about stopping for a drink from a canteen or a bottle, it's just a little smoother, and it's a nice big carrier.
Thanks again people!