All else is normal. Eating well, pooping and peeing okay, appetite fine, he just is not his normal, bouncing around, nose into everything, self. He just lies there and sleeps. This all came on very suddenly within an hour or so of his monthly heartworm med. Never happened before with HW med.

Call your vet's office and explain the sudden change in your dog. Since they would have his medical history, they should be able to give you a better idea of whether it is a reaction to the heart worm medication.

When was his last check up and screening for worms?

Were there any changes in his environment about the time you gave him his medication? Elevation, temperature suddenly going up (such as being used to 70 degrees and it is suddenly 90 degrees), humidity, people, new animals, etc.