Assuming this would be meant as a joke

[DingDangBang]

DBIL hasn't had a proper job ever. He thinks he's going to get discovered. He's 36. He's not (though he does get occasional work in his field of choice). This has meant that he has very regularly relied mainly on other low paying work to make ends meet - along with a general attitude that others will help him out if he gets into trouble.

We have quite a lot of money behind us due to a mixture of luck (inheritance on my side) and a very successful business. He quite often says in front of us that he can no longer do this or that as he's no longer got x or y to spare.

This evening, our DS asked (through DH) if DBIL could come and see him (they get on well which is nice). DBIL's response to DH was that his car was in the shop, will probably need to be replaced and that he would come and see us if we bought him a car. Not sure if he was joking or not. What do you think?

Plenty of aspiring singers/artists/whatevers work while they pursue dream jobs. Why can't he?

Waiting to be discovered?
Hope he’s not an aspiring comedian.

Just laugh heartily and say "Oh BIL, you're so funny!" if he is hinting in future. Friendly and affectionate and making it clear you won't give him a penny.

Maybe he should focus on taking care of her instead of trying to scrounge car money off you. He might try and get money for prescriptions etc from you and there might well be no illness at all. Don't give them money, just respond to any sob stories with, "I'm sorry to hear that" or "Oh dear" or "That's a shame". Something sympathetic but non committal.
It's unfortunate that BIL is like that but not you or your DH's problem nor responsibility.

If he asks for money again tell him you know a way he can get recurring payments every month... he could GET A JOB.

Yeah he won’t get jack shit from me.

Tell him on his bike. Literally