Welcome Gene! Bettas sometimes need a little time to get adjusted to new surroundings. So he could be in his adjustment phase. My bettas have usually only taken a few days. If you just put him in, give him some more time.
About the filter. I think it's a good idea to have one. But you are right, the flow might be bothering him. You can use this tutorial
to show you how to baffle the flow from the filter using just a water bottle. I have this type of filter baffle in my goldfish and 29 gal community tank. To prevent his fins from getting sucked up in the inflow, you can use a clean rubber band and attach an aquarium sponge to it.
Hope your boy adjusts soon!