Sapphire did the same thing when she tried to jump over the side of the slide (she was standing at the bottom). We thought she might need to go to the vet because of her behavior: she yipped, and when we went over to help her get up and go indoors, she started snapping everywhere (from pain) and threw herself on the ground - she wouldn't even get up at first. It was pretty scary.
However, she did get up with some persuasion, and we managed to get her inside and made her rest in her crate with a chew. Between that and an anti-inflammatory pill, she was fine within a couple of days and has had zero related issues since. Had she not shown improvement in the morning (this was in the evening), she would have gone in to the vet for x-rays.
So, Angus may just need some crate rest. I hope he feels better soon.
Sofia