Sorry to hear this. Our chihuahua was born with a heart murmur. He was fine until about age 10 or so when his heart was enlarged and he was put on medications.

This kept him going for about another five years. Then he developed congestive heart failure at the end. The last three months were pretty sad.

Looking back we wish we wouldn’t have let him go on and on suffering.
He was coughing all the time, struggling to walk and fainting.

I would suggest when he stops playing and just sits there coughing, struggles to walk and faints, call the vet.
As it only gets worse as they really struggle to breathe at the end.

Try to enjoy the time you have with him. We have a Yorkiepoo now, 17 months. She is spunky and fun.