Regardless of the species of pleco, a 5 gallon, let alone a 1 gallon, is wayyyy too small. Bristlenose are the smallest, only getting 3-5 inches. a 10 gallon is needed for one and a 20 is needed for 2. A common pleco needs at least a 55 gallon. Too small of tanks can cause stunting or stress. And stressed fish are much more susceptible to illness and premature death.
I would find someone to take him. If you know anyone, or try craigslist. Or bring him back to the store. And wal-mart is just about the worst place to buy fish. They know absolutely nothing, will never give you right information, and, as a whole, take the worst care of their fish. See if you have a local or chain pet store around for buying fish. And always do research before bringing an animal home. Our sister site tropicalfishkeeping.com has fish profiles with good, concise information on tons of species, it's a great place to start.
Please don't just stick him in a 5 gallon. It's too small. Either take him back or find him a proper home.
Tanks- 55 gal(in progress)-Common pleco, Angels, lemon and rummynose tetra, blue rams; 29 gal planted- rasboras, corys, otos, clown plec, shrimp; 10 gal- juvenile goldfish; 6 gal planted- betta
Other Pets-Maggie(Tricolor Paint Mare), Lily(Sun Conure), Zoe and Melina(Budgies), Leonardo- Bearded Dragon, Pilot and Phoenix(Golden Retrievers), Beckham(german shepard/akita), Myrtle and Richard(hermit crabs)
A Better Betta Fish Care Video!