I know a few pilots. They love their job, it pays (or did) well. They work hard. But for most people who want to be pilots, it's a dream they can't achieve because of the cost. Or the cost scares them off.
I think your son doesn't appreciate how lucky he was, to not only achieve his dream. But to have a well paying job, without the debt to pay off. He has had all the positives of the job, but not big the big negative of handling the debt. He only got the good bits.
What's happening in his industry, is shit and its great he got a job. But again, as you are looking after him financially, he isn't facing upto what's really going on again. He is used to you picking up and fixing anything financial that holds him back.
In his head, the 35k is easy come easy go, because he is assuming he will be back at work and will have another 35k, in less than a year. And if he doesn't and spend the money. He will still be ok. Isn't going to think about being sensible with it. Because he doesn't need to.
The back lash at you, is because both you and his girlfriend wouldn't tell him what a good idea it was. You didn't side with her. You both had an independent opinion that happened to be the same.
And yes, it's been a shit 18 months. So yes he might be wanting to do something to make him feel excited and cheer him up. But there's loads he could do, to give himself a pick me up, that is so much cheaper.
He needs to accept he isn't a pilot at the moment. He can't live as though he has a pilots wage and needs to think about the future. But he doesn't need to really, because you will pick him anyway.
And it must be difficult. If I had the cash, I would probably my do similar. I think most people would. Who would see their kids struggle when they have the means to fix it?
Also remember a bit of the anger he is directing at you is, probably, really aimed at the whole situation and covid in general.
But things need to change. It's not ok for him to do this.