Alexander had only known Philip at this age for a short time, but already he was sure that the boy was not one to be prone to hysterics over nothing. Philip was breathless from running, and there was a panic and an urgency about him, and... yes, the younger boy was not with him.
Alexander got up immediately too, even if his son hadn't specified that he should come, he was hardly just going to sit and drink tea and let Black get on with it by himself. Philip led the way quickly, running a few paces ahead of the grown up and calling "he's coming!" towards his friend.
Once Mr Black took over, Philip stepped back and bit on his lower lip anxiously. His father caught up with him and reached out to give his shoulder a small squeeze so he'd know he was mad. Whatever had happened, at least he'd done the right thing by running in for help. Philip looked to his dad first, who gave him a little nod of encouragement, and then he started.
"Well, sir... Sirius wanted to show me how the broomstick works. He was really very careful, and showed me all the safety checks. He was just flying slowly and not very far, so that he could show me. And then he accidentally slipped and couldn't hold on," he explained carefully, but clearly.