Your broader philosophical question is, I think, an easier question to approach from a philosophical view point.
"What are one's obligations when interacting with someone who may be too generous for their own good?"
Different View Points
Depending on the utilitarian you are most likely morally right to continue to ask for help as long as the following are true.
The unhappiness you would suffer from not asking for and receiving the information is greater than the unhappiness he is suffering from providing the information.
There are no other options that would produce greater utility.
The first criteria is probably fairly easy to assume. If the person is continuing to reply than obviously they are not suffering greatly as they are under no obligation to reply. One could even argue that people who posses knowledge often times give away their knowledge for free because it makes them happy, so this situation could be mutually beneficial. On the opposing side the person could be unhappily giving out information for free in which case you would be morally wrong to continue to ask them for help (provided #2 is true), however interestingly it is also morally wrong for them to continue to give out information if their unhappiness is greater than the unhappiness you would have without the information (again provided #2 is true). You would have to weigh all the exchanges you've had with the person to determine what you think their happiness would be and also examine what your happiness would be without the information in order to determine if the first criteria is true.
The second criteria is important because it is most likely where your morality is called into question. Provided that the first part is true you are morally obligated to look at all other options, such as enrolling in a class, purchasing a book, or even hiring a personal expert on the subject. All options have to accessed. Then within each of these you must consider the happiness of all parties involved. This may seem daunting however their are a few quick questions that could put to rest most of these scenarios. Most likely you are morally right if the other person is gaining happiness through knowing their own charity and possibly living vicariously through a young mind. You are most likely immoral if you could just as easily (if money isn't an issue) enroll in a course and stop pestering someone who has better things to do.
For more info on utilitarianism see the wikipedia page and look for instances of the greatest happiness principle.
A Categorical Imperative
If you believe in Kantianism than you must follow these steps. (taken from wikipedia)
Act only according to that maxim whereby you can at the same time will that it should become a universal law.
Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
Therefore, every rational being must so act as if he were through his maxim always a legislating member in the universal kingdom of ends.
In essence you would have to believe in something like this: "It is morally right to continue to ask for something as long as the other person allows you to." and believe it should be a categorical imperative.
Bits of Classical Liberalism
It's important to note that you are making a very big assumption in the second half of your question. You propose the idea that you are able to judge whether or not someone else is "too generous for their own good". Classical liberalism calls into question this idea, in terms of government, whether it is up to you to impose your view of the good on another person. So you could conclude that the only obligation that you have to the other person is that you always give them the right not to answer you.
There is no set answer to this question it has to do with what you believe. I hope that you can agree with one of the above points and that one of them answers your question.