Betta laying at the bottom of the tank… unless I chase him?

Unfortunately I don't have any pictures on hand, or any info re. Water readings but want some advice. He literally just lays there unless I come near him, and if I do he'll constantly speed away until I leave (so he clearly can swim). I unironically mean he doesn't move all day, and I fear he isn't even eating. He's in a 5 gallon with conditioned water.

I know nothing about bettas, I just took him in from a friend bc they were going away for a month and were going to leave him to die, so looking for some guidance please! I got him in a tank that was probs a gallon and he started to decline, so his new tank is less than a week old. He's been laying at the bottom of both tanks, though.