Cassian jumped from the horse without waiting for it to stop properly. His feet staggered a bit but he did not stop.
He ran inside the sweet shop where his son should be. He should have accompanied him. He should have been with him.
But he did not want him to go. So, he tried to stop him. When Alec did not listen, he told him that he would not come.
He had thought due to his timid nature, the child would not go. But he was wrong! His will to meet Mary was much stronger than his fears.
He also realized that he was not as close to Alec as he had thought. If Alec would have asked him to accompany him, Cassian would have done it with joy.
But that did not happen. He did not even ask once if his father was coming with him. When he ran towards the carriage, it was already left.