i have noticed it only happens when he’s sat down
It could be normal. Goslings metabolisms fluctuate wildly when they’re little. One second they’re too cold, the next they’re too hot and you have to be quick about adjusting everything to their needs.
Or it could be something like a lower respiratory or digestive infection. If the rear airsacks are inflamed from an infection it can cause them to pant when they lay down, their abdomen will be really tender and sore because of this and they’ll often be reluctant to lie down.
A lower digestive parasite or infection like coccidia, some bacteria etc, can cause the intestines to be inflamed which will cause pressure on the lower airsacks which causes breathing problems that look like a lower respiratory infection also.
You can look at his droppings, if you see unusual colors like black, pink, orange, red, yellow, excessive white or bubbly that’s pretty typical to see in a lower intestinal infection from parasites or bacteria. They’ll also be more reluctant to eat because they feel awful.
If he’s behaving completely normally and eating normally then he’s probably fine.