Without question, your dog needs to be seen by a vet as soon as possible. There are a couple big things that could be going on here:
First, your dog could have ingested a toxic substance (the snake repellent) and is suffering a reaction from that. Many poisons damage the kidneys or liver after ingestion, so the shallow breathing and lethargy you're seeing could absolutely be from that. Without treatment, most ingested toxins are fatal.
Secondly, the hot piece of meat could have burned his esophagus or the digestive tract. The breathing you're seeing could be a sign of severe discomfort. Having the dog evaluated, to ensure that he's not suffering 3rd degree burns in his throat is the best thing to do. Left untreated, these kinds of burns can actually cause the esophagus to swell to the point of cutting of air flow in the trachea, which will cause the dog to suffocate and die.
Please get your dog to a vet as soon as you can, first thing this morning.