I've found that as people, especially men, age, they can get very set in their ways and stubborn. My father insisted on attempting to undertake things in his late 80s that he shouldn't have been doing anymore ... just because that was what he did. He developed behavior patterns that it was impossible to shake him out of.
Now, your primary responsibility is to take care of your own family financially, and you have an obligation to take care of your parents ... in terms of the essentials. So, if it gets to the point that your parents can't afford food, medicine, clothing, or other essentials, then you need to try stepping in. If he won't be persuaded from selling off pieces of the farm to maintain his lifestyle, then there's nothing more you can do. I feel that you would be doing wrong to your family to give your parents money so that they can "buy presents" and the like. Buying food and other essentials would be a different story. So I personally feel that you should NOT give them anything. You'd be taking away from what you are obligated to give to your own immediate family in order to give your parents something that you are not obligated to give ... extra spending money.