He is a child. Children make mistakes and it is a parent's job to teach them how to avoid the common ones.
It will have happened because he was tired/ distracted and his brain went into automatic mode to just pick up and register the weight of the one bag he carries most days.
If you punish him, you'll just end up with a child/ teen who doesn’t confide in you. It’s concerning that this is your train of thought.
Have you taught him to count the number of items he is carrying when he gets on and off the bus and practised with him since he was young?
Have you also taught him to look back into his seat and floor space to check he hasn’t left anything behind?
If you haven’t, then it is you who should carry the “punishment” for not teaching him. You are the adult, with the fully-developed brain, several decades of life experience and the responsibility for teaching your child how to avoid making mistakes like this.
If you have properly and explicitly taught and practised these skills with him and he forgot, then take this once as a mistake, replace the shoes properly, and ask him what contribution he should make if it happens again.
If it keeps happening, then you need to rethink strategies e.g. he needs a school bag big enough to carry everything in or reminders on his phone timed to pop up at certain times or locations.
If he did it deliberately, then you have bigger problems than a pair of lost shoes and you need to unpick why he did it, e.g. to avoid going to school or to get a replacement pair for some reason.
Some of the responses on here are just unkind and there are probably posters enjoying the thought of a child being punished at their suggestion for a mistake.