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:
- You wouldn't think that such a tiny cute dog would be so ferocious, but in fact, it's very dangerous.
- I didn't eat breakfast, but I'm much less hungry than I thought I'd be.
- After moving to LA, I learned that seeing a celebrity at the grocery store isn't as exciting as you would think.
"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.