The causes of cochlear hearing loss are multiple and diverse. Loud noises will do it for anyone, but many people have one of a dozen-odd kinds of non-syndromic hearing loss that are generally due to single point defects (Single Nucleotide Polymorphisms or SNPs) in the gene COCH, which codes for an important cochlear protein. There are also multiple multi-gene or developmental syndromes that result in cochlear hearing loss (not all hearing loss is cochlear, although a great deal of it is.)

Most cochlear hearing loss is symptomatically similar, although a degree of disorientation is more common in non-syndromic loss due to SNPs, but because the causes are so diverse what works for one person--even granted there are such things--will be completely useless for most others. It is very important to keep this in mind, because you don't appear to have more than incrementally useful evidence that your son's milder tinnitus is due to magnesium rich foods. A single case is simply not very useful for increasing the posterior plausibility of the claim "X causes Y".

Remember, reasoning is fundamentally Bayesian, and Bayes' rule is:

p(X|E) = p(X)P(E|X)/P(E)

where in this case p(X) = prior plausibility of claim X that "magnesium rich foods reduce tinnitus", P(E|X) = probability of your son's tinnitus being reduced given X is true and P(E) = probability that your son's tinnitus wouldn't be so bad anyway. p(X|E) = posterior plausibility of the claim X given your observation that your son's tinnitus is not so bad and he ate lots of magnesium rich foods.

Your prior p(X) will be different than mine: plausibilities are subjective, unlike probabilities, which are objective... it could hardly be otherwise as plausibilities are degrees of belief and you can't have a belief without a believer... that is to say, a subject. But what matters is the ratio P(E|X)/P(E), the ratio of probabilities. Let's assume for the sake of argument that P(E|X) is 1-&epsilon;, as high as it can be. If "magnesium rich foods reduce tinnitus" is true, your son's hearing would almost certainly get better.

But here's the question: what is the natural variablity of tinnitus? That's basically what P(E) depends on. If tinnitus varies a lot due to time or circumstances, the P(E) is also going to be pretty high (well aboe &epsilon;, at least) and therefore the posterior plausibility p(X|E) is not going to go up by much (a factor of two is incremental: interesting arguments and evidence involve many orders of magnitude increases).

So although it isn't obvious (and forgive me for belabouring the point... I just got hooked on this example) when you claim "it was magnesium rich foods that helped" you are actually asserting "I know what the natural variability of tinnitus is", due to the argument above.

And my question to you is: do you? If so: how?

I've gone through this in detail here because this kind of reasoning is incredibly common, where a person pulls a single fact out and presents it as "proof" for some proposition (and you may not have meant to do that here, but it's what I'm reading into your words and I don't think I'm wrong in suggesting others will read in the same thing... my apologies if I've taken you wrong!) When someone says, "I did X and Y happened so I know X causes Y" they are saying, "I know what the natural variability of Y is".

I would encourage anyone on the receiving end of such an argument to ask them what the natural variability of Y is, because if they do not have that knowledge, they do not have a legitimate--which is to say, Bayesian--claim on the conclusion they wish to draw.