Getting a son after a long period of time, made Dhritarashtra emotional n so, he can't say no to Duryodhana in any matter. He even gave his kingdom, Hasthinapur when his asked for it as a materialistic thing in the game. As a father he was very happy for his son's success. He wanted his son to be the best and the ruler and so, he supported Duryodhana's maternal uncle, Shakuni too to use unfair means to let his son win the game. And taking advantage too of his father's Duryodhana blackmails his father sometimes.