He's a lucky pup to have you. A lot of people might not have noticed something as subtle as pale gums and a little bit of lethargy.
We have a dog here that has early stage heart failure. She developed a cough and some lethargy and they diagnosed it when we brought her in for a checkup. She's doing quite well on the meds. Hopefully, that will be the case for your little guy, too!
Keep us posted, pls, on what they find out!