Well, I would say that your son is not addicted to two products. He's added to one thing, which is nicotine, and he's finding it from wherever he can get it, so the more alternative sources of nicotine there are out there for him, the better.
Being a smoker is like being trapped in a nightclub when a fire breaks out. You need as many exits as possible, and it doesn't matter which one you use.
However, dual use is common with these products, just as dual use is common with medicinal nicotine. But what we do know, and the British National Institute for Health and Care Excellence has accepted this in its guidance on harm reduction, is that dual users are much more likely to quit smoking completely than people who never experiment with an alternative nicotine product. So the outlook for your son is good: he's going to quit, but it may take him a year or two to get around to it. That's the reality.
The strength of electronic cigarettes is that it draws people like your son to try nicotine products, people who otherwise wouldn't. The history of NRT is that it's used by a small minority of smokers, and electronic cigarettes by a much greater proportion.