He had to differentiate for himself between what Bill told him and what his Uncle Andy told him.

He was a serious-minded child, who had already passed through that most painful period of doubt as to Santa Claus and the Fairies, and had not yet reached the period of certainty about everything.

He was capable of both belief and doubt.

So, naturally, he had his difficulties.
Bill certainly knew an astonishing lot about the creatures of the wild.
But also, like all guides who are worth their salt, he knew an astonishing lot of things that weren't so.

He had imagination, or he would never have done for a guide.