I would let this sit a while with him but make sure he knows that this is not the end of the matter.
I’d just say to him that trust is really important in a family, particularly with people you love.
And if he doesn’t admit to it, that casts suspicions on the whole family, including his little brother, and he is meant to be setting an example. (Make sure he knows that you have questioned his little brother about it too.)
And it’s very important that you are able to trust his word but even more than that, he mustn’t ever pretend to delude himself about the truth. He needs to be able to trust himself and his actions most of all, because that is what being an adult means.
So tell him calmly that he needs to go away and think about this very seriously.
Then say you will discuss this with him again next weekend because it’s important to you that you know where that amount of money has gone. You can’t ignore the loss of £70 as it is hard earned money.
As a teen his impulsivity is already high and if he is ND , then that could affect him in a way that makes him more susceptible to his impulses, more likely to say the first thing that comes in to his head, or indeed steal the money or buy the snacks in the first place, so I would give him a little leeway.
But say that this matter is not over and obviously if you can’t trust him to tell the truth, then there will be natural consequences to that because the more trust there is between you, the more freedom he can have. But if you can’t trust his word, then you may not trust his word about other things, such as how long he will be out with his friend or how long he will spend gaming etc.
Just let him stew and ponder a bit without any pressure and set a specific meeting time to discuss it, say next Saturday at 2 pm, to show you are serious and to focus his mind, but obviously he can come and have a private word with you or his dad any time leading up to that point.