How far away does he live? What kind of $$ are involved? Can you afford to sacrifice those dollars with little or no expectation of repayment?
Apart from this low point of financial need, what kind of relationship do you have?
Are you pals? Do you watch the ball game together? Sit on the porch sharing cold beers and discussing each other's work/career/dreams?
If you answered yes to the last three, great, you already know what to do.
If you wish you could say yes, then you have an opportunity right now to strengthen your relationship and make it more rewarding.
If a quick substantial cash injection will take immediate stress out of his life and you can do it, good on you. If this is more an ongoing problem due to the low-paying job, maybe frequent small fiscal boosts could be a way to help out too.
Try creatively linking family bonding to fiscal support, with the focus on family bonding. Spend more hang time and while you're at it pick up some mundane costs for him, boring, basic stuff, thereby letting him devote his cash flow to more luxury items (like taste in clothing).
Invite him over to watch the game and when he goes tell him to take the rest of that case of beer along. Or stop by his place with the beer and shoot the shit while you christen the case. May it's not always beer, maybe take him steaks for the BBQ. Or get in the habit of treating him and his girlfriend to a nice dinner out ... with you (+partner if attached).
The key is to make this process of supporting him in little ways be the side effect of getting together. Done crassly it's just bribery for affection, especially if you haven't been close. Done more naturally it's just fatherly generosity. You might not solve all his financial problems, but this will be way more pleasant for both of you than preaching at him.
All of this getting together is premised, of course, on the assumption you both live in the same city. If you're at distance, you may still want to help him with cash input from time to time, and I would recommend it be accompanied by "family time." Go visit for the weekend. Have him come to town and bring his girlfriend. Take them out to experience live theatre. Treat them to dinner. Take them on vacation on your tab if you can afford it and think you can survive a week or two of togetherness. Do what you can. I would.
Your son sounds like a good kid. You should be grateful he calls to vent.
My son is of similar age. He calls to vent all the time. Or talk about work. Or just to shoot the shit. Or we help each other with home reno projects, during which we may grumble loudly at each other. Sometimes we grab a sandwich at lunch, sometimes a beer after work. I did not experience this kind of relationship with my own father. I am grateful to enjoy it with my son.
Sometimes I look at my credit card statement and think, "How did this get on my bill? What's the chance of extracting that cash from his pocket? Will there never be an end to the financial cost of parenting?"
And then I think "What a small price to pay for this friendship."