How to deal with TimBob's worry?

I don't think you should stop answering questions now because if you do, he'll think something has changed and things are no longer okay. But perhaps you can become more vague and not let on that things got worse. Are there books about kids who's parents have what you have? Maybe if he can see another kid going through what your family is going through he will be less worried. But I think the curiosity is natural. It's something wrong with his mum, so of course he wants to understand it. I think honestly explaining it was a good way of dealing.