If it isn't safe, buy it bottled. If you don't like the taste, man up and deal with it; that much sugar is insanely unhealthy.
I'm no dietician (and to be honest at my university us from the other paramedical studies snicker about the dieticians because they are pretty odd folk by and large), could you explain, or link me to somewhere, why sugar is so unhealthy? I'm not trying to wind you up, I genuinely don't know and am curious. Isn't it all just a form of carbohydrate in the end?
The gist of it is that sugar isn't innately unhealthy in a moderate amount. Your body does need glucose after all. The problem with two litres of sugary drinks a day would be the over 200 grams of sugar, on top of whatever else he might be eating throughout the day. Even a non-low carb diet shouldn't involve much more than 300 grams of carbohydrates a day. If 2/3 of your daily carb intake is coming from a nutritionally worthless substance, that leaves less room for nutritionally valuable things like fruits and vegetables. That's actually the real danger with any sweetened drink(including fruit juice): they provided calories, usually a lot, but don't satiate you at all. Makes it very easy to over eat.
So I guess that's why sugar is so unhealthy: it doesn't satiate and makes over-consumption very easy and likely. Diabetes is also a concern for many people, especially if they're over-weight. Also possible links to higher LDL(bad cholesterol) levels, but I'm haven't been following that.
I'm having trouble finding specific info that doesn't go on about "toxins" and other biological boogymen. I'll link here
for now. It's a copy of the Low Carb Megathread originally from Something Awful's Watch and Woot forum. Some of the information is out of date, but the basics are useful. I don't personally recommend low-carb. It works, but not generally for the reason proponents think.
Also: do NOT rely on your metabolism. It will betray you eventually. I'm a pretty small guy in general, and when I started college, I was tiny: all of a 120 pounds. But the abundance of cheap, shitty food and the lack of activity caused me to gain a good deal of weight. It happened gradually so I didn't really realize it. But by the time I graduated, I had gained almost thirty pounds and a chin. It kept going up from there until I did something about it. Keep exercising, but don't get complacent with your diet.