Welcome to PT and congrats on the new Betta!

I've had a couple spawns and have had well over 100 bettas come into this house. I had over 40 seperate tanks at one point. I've since stopped bringing home new bettas because the three dogs keep me busy enough so now it's just two.

Since you got him last Tuesday and it's now Sunday, he should be settling in and getting comfy in his new home. If he's still not eating, try a different food. I personally only feed Hikari frozen bloodworms or Hikari Betta Bio-Gold. Those two seem to be favorites for both owners and breeders. I have never had a betta pass up frozen bloodworms. They love it! Try that if he's still not eating.

Don't worry too much about him sleeping. It's nice if you can provide some things for them to rest on, like plants, just so they can rest their fins for a bit.

As for what the girl at Petsmart told you..

-DO change the water at least once a week. Preferably 2 (or even three depending on if he acts stressed out with that many a week). One is the absolute bare minimum. If you don't have a cycled tank (I won't cycle a tank under 10 gallons), then do 100% water changes (take the temp of the old water, put him in another container, match the new water's temp with the old water's temp, add dechlorinator, aquarium salt if you use it, and net him back into the tank)

-Moving water tends to stress them out

-You don't have to have a live plant. Bettas go to the surface for air so they don't need a plant, air bubbles, etc. Live plants are great for them to rest on and play around on though

-Add gravel if you like. It doesn't really matter. I didn't when I got more bettas because it became a pain to do 100% water changes for all of them. If you do choose to filter a larger tank though, gravel is a must.

-Decorations and fake plants are nice so they have some place to hide.. but be careful because their fins rip very easily. Avoid the sharp plastic plants.

Good luck! Bettas are LOTS of fun!