Get Your Dog Care Questions Answered by Experts
I am sorry to hear you are worried about Ranger. I am a veterinarian and will do my best to help. It appears Joan is not available. I can help you, or I can opt out and you can wait for Joan.
Fainting episodes are often due to a heart problem. I would be concerned that the fainting might be related to the heart murmur. Were any chest Xrays taken? He might have something neurological, but these could be fainting, from lack of blood of to the brain.
I would look at his gums the next time he faints. If this is from a neurologic problem, his gums probably will remain a normal pink color. If it is from a heart or circuation problem, his gums might look pale when he faints, and then turn pink again as he wakes up.
It is probably safe to give him the melatonin, but it probably won't prevent or affect the fainting episodes. I would start with a low dose, 3 mg.
Let me know what else I can answer or help with.