You mentioned "something that he can carry with him", does that mean in a pocket (either clipped to his pocket, or just dropped in), or are we talking about carrying the knife in a belt sheath? How does he prefer to carry his knife?
Many folding knives, including some referenced in this thread (like the classic Buck 110 and 112), are a bit heavy, and in my opinion and experience (I've owned both of those) are not well suited for everyday pocket carry. This might be an important consideration.
Also, knives small enough to be easily carried everyday in a pocket aren't likely going to make good fishing knives. Not if we are talking about doing things like filleting fish. As for things like cutting line, any size knife will work.
Another thing to consider when buying an expensing knife for fishing is- knives used for fishing can easily end up at the bottom of a river/lake/pond/ocean if they get dropped when someone fishes from a boat, dock, or pier. That is one reason why I would suggest inexpensive knives for fishing.