Is a son's refusal to buy a house he does not want, at his father's request, considered disobedience to him, given the son's knowledge of the difficulty of acquiring money, and that his siblings cannot help him with the purchase?

Purchasing a house for your father is not an act of disobedience if you do not need it yourself and it would be a hardship for you, as obedience to parents is not absolutely obligatory. It is best to seek guidance from Allah (istikharah), and after performing istikharah, you may find yourself inclined to fulfill your father's wish. If you insist on refusing, then be gentle with him in your response and apologize to him with kind words and good deeds. Maintain ties with him financially as much as possible, even if he is not in need.