Punishments do NOT work, and are usually counter-productive. If you punish your son, he'll simply find ways to be more sneaky and underhand.
Far more useful would be to sit down with him, tell him you love him (and will always do so) and then say that you're concerned that he's started this habit. It's pretty unusual for a 13yo to start smoking unless it's something encouraged by other family members - so if you or any of your relatives smoke, you're on very shaky ground. It's a disgusting habit as well as being unhealthy (and expensive!) but saying that to a teenager isnt going to help in the slightest. He needs to see it for himself, and if it's something he sees parents/siblings/aunts/uncles doing then you're going to have a hard time convincing him not to. And I'm assuming that there must be someone in your home who smokes, otherwise you would have noticed it instantly he started - the stench of cigarettes is so strong (and vile) that it's impossible to disguise under normal (non-smoking) circumstances.
However, if there really isn't anyone in the family who smokes, then it's more worrying from a psychological perspective. Either he's seriously passive aggressive or he's succumbing to peer pressure in a very bad way. That makes him an insecure troubled child.. who needs a lot of love and attention - again, NOT punishments.
So don't criticise, or judge, or scold him. Just say that you're concerned about his potential health, and perhaps about where he's finding the money to smoke. Say that you understand that if friends do this, it can be hard to say no. Wait for him to talk - he should be able to talk about anything without criticism - and see if you can figure out a way forward together.
I'm sure he knows all the health problems associated with smoking - schools talk about them all the time, and he's probably well aware, but doesn't see them as relevant. So don't try and talk about them or your son will think that you consider him stupid. Instead, get to the root of what has caused this, and let him know that you're on his side. He would probably love to give up, but will only do so with your unqualified and unconditional support.