Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Since you noted that he is breathing heavy, can you take a breathing rate (just count his breaths for 10 seconds + multiply by 6)?
If you press on his belly, does he have any discomfort, tenderness, or tensing?
Any gagging, retching, lip licking, drooling or vomiting?
Are his gums pink or pale/white? Moist or sticky?
Could he have eaten something he should not have (ie bones, toys, plants, chemicals, etc)?