he's assured me lots that he has.
That's the problem with a liar; they just get better at it all, including lying to themselves and everyone else.
Makes you feel like a failure as a parent.
That's not true, though. Nothing you did encouraged this.
It is difficult. Does he know you know? You may be able to assess how serious this is by sitting him down and asking him to be honest about the laptop. If he sticks to the lie; I'd be more likely to seriously consider the police. If he comes clean, and is very honest, that may suggest that he's feeling guilty at some level.
When are GPs due back? You really need to secure that card... It's likely he's got the details written down somewhere.
I think it'll be a tough decision what to do here. I tend to run on the side that the police can be heavy handed; but this is really serious and as he's had some issues with the truth before, I wonder if it's worth risking a caution to drive home that this isn't acceptable and hope it stops him in his tracks. If it's reported as fraud to the CC company; they could report it anyway.
I had to report my little sister for theft; it started with nicking £10 notes from me or my foster mum, then £25, then she'd steal any money that she could see. Then she started stealing cards and drawing money out in the branch, the staff knew us so believed her when she said our foster mum had sent her to draw out £X but she'd forgotten the PIN. Then she started ordering online. Eventually she'd steal any money she saw; it was like a compulsion. Because she'd got away with it; and because she did it so much, any money she knew about felt like 'hers'.
She was interviewed and came clean, the police threatened taking her to court, a duty solicitor explained what that would do to her prospects. They let her go in the end; and as far as I know, she hasn't done it again. She was 15 too. I won't say it wasn't immensely stressful, but I hope it stopped it escalating, because it seemed pretty certain to before. And people still struggle to trust her, even though she's now 23.