Novel: When his critically ill mother tells him to stop drifting through life, a penpusher leaves his entire life behind to go on an adventure and pursue his dream of finding a legendary animal in the heart of south america.

I don't care for "drifting through life."

When his mother falls critically ill, a secluded son undertakes her life-long quest to find a legendary animal located in the heart of South America before she dies.

So how could I make this work? By changing the reason he wants to go on the adventure and forget about the mother?

So basically, it's the deceased mother's story? That's a red flag.

Foxtrot25,
I had in mind the fact that he and his mother used to study stories about this legendary animal and they used to go on adventures together. But then life got in the way and he followed the beaten path with a college degree and a boring job.

So when she dies, he finds that she kept all their "studies" on the legendary animal and decides to honor the memory of his mother (not sure that was clear) by searching for it.

Jumeriot,

How does the death of a mother want to make a guy want to chase after a cat? Even further, how does one honor the memory of a wild cat?

Focus on the adventure and why the MC needs to partake in it. A pencil pusher may not make the best adventurer.