This is a classic example of "morbid humor".
"Set fire to X" means "cause X to start burning". So, if you set fire to a man, you are burning him, and he will most likely die in very short order.
It's a play on the old saying "give a man a fish and you've fed him for a day; teach a man to fish and you've fed him for the rest of his life."
I'm not sure I have a definitive grammatical answer, but in my experience as a native American English speaker, the "didn't" variant is more correct and natural. Saying
I haven't heard from him.
implies that although you have not heard from him yet, there is some expectation you might hear from him in the future.
Whereas
I didn't hear from him.
is strictly talking about what is in the past. By using
I didn't hear from him until now.
you are adding contrast between the past and the present.
I suppose "I haven't heard from him until now" might be ok, but it seems like the "until now" (aha, something has changed!) conflicts with the hopefulness/tentativeness of "I haven't heard from him" — how can you still be hopeful if you already know that the situation has changed?
re: "until now" vs. "till now" vs. "up to now":
"till" is an informal variant of "until". (although The Free Dictionary says that till
came before until
historically) If you say "up to now", people will understand you, and I don't think it is grammatically incorrect, but it is somewhat unnatural-sounding. I think the reason is that "until" is such a common preposition and has a very good match for this situation, that alternative forms don't seem right.
Hope this helps!
Best Answer
In this context, "You wouldn't think it" means "you would never guess", and "to look at him" means "based on how he is now". So the sentence as a whole means that Jack doesn't seem like the kind of person who would be bullied in school, but in fact, he was.
In general, "you would think" or "you wouldn't think" refer to making assumptions or having a general feeling that something is the case. Often this assumption or feeling is contradicted in the rest of the sentence as in your example. Some other examples:
"To look at him" or "by looking at him" can refer to a person's physical appearance, but often (and possibly in this case depending on the rest of the context) can refer to a person's general context. It may be that Jack is very physically strong, indicating that it's unlikely a bully could hurt him. It's also possible that Jack is very confident so not a likely target for a bully.