Well shame on him, a big ole 36 year old baby borrowing $ from his parents cause he cannot manage his own money. People, especially family members, treat others as their personal ATM machine because they can.
Repeatedly lending him money to bridge the gap between paydays, achieves one thing....he expects you to bail him out of his problem.
Whenever I have a problem with the NO word, what works for me is to remind myself that NO is a complete sentance. If I venture just one word beyond NO, I open the door to negotation and we all know where that leads.
No need to rationalize, justify, get into his business or your own financial situation. One word, NO, will achieve your goal.