This is a bad idea, both for practical and legal reasons. It is also, in my opinion, rather disrespectful to impersonate an employee, but as your boss obviously sees this differently, it will probably not help to argue that personal point.
However, you could ask to talk to your boss in private, and point out the problems with his approach.
Sending mail under someone elses name will look shady and questionable to many people, so there is a real risk the correspondent will take this badly if they find out. This could happen as easily as the correspondent phoning you to ask about the mail, and you saying "I did not send that".
In addition to that, the other side probably does not care who answers, as long as their problem is handled - so impersonating you creates a risk without a benefit.
The details will of course depend on jurisdiction, but impersonating may even be a criminal offense. Just writing a mail under someone's name is probably not criminal, but signing a document (such as a letter with a binding offer), or convincing someone to do something because they trust the sender might well be considered forgery or fraud. While an actual criminal case does not seem likely, why take the risk?
To reach a solution, it will probably help to understand why your boss did this.
- Was it just for technical reasons? Maybe the mail was in your mailbox, and he did not realize he could copy/forward it to his own to answer it under his own name? Then resolve that. Maybe a shared customer service mailbox is the answer?
- You write that your boss used your laptop to send, because his own was not available. This makes it sound like a practical problem. Try addressing that. It might be as simple as giving him an account on your laptop, with his own mail configuration.
- Did he not want the correspondent to know you are currently not available? Then try to find a solution for how to communicate this.
- Something else entirely?
Finally, if your boss does not show any signs of wanting to work differently, you'll have to weigh your options. Finding a different job may be your only option.
Also, you might consider explicitly forbidding your boss from sending mails in your name, if your jurisdiction allows for that (in Germany, for example, this is your right, BGB §12). However, you will have to consider how that will impact the relationship to your boss.