Assuming he'd agree that advertising was immoral, which he arguably would as advertising manipulates people in a way that might not be in their best interests, thereby using them as means and not as ends in and of themselves.
It's an interesting case though as merely informing people is not immoral, far from it, you have a moral duty to spread information that you think would help others. The problem with advertising is that it's not furthering other people's goals, but instead uses them as means to further the advertisers goals, which violates Kant's principles.
In my eyes - the only morally decent advertisement are advertisements that are solely education of a product or service you may need exists. Such as a boring infomercial without the manipulative sense of urgency "buy now and receive blah blah blah". But because these ads aren't manipulative of the human psyche, they would be terrible at driving sales to the product/service especially when competing with advertisements that are manipulative. Which is why nobody makes ads like that - they all rely on manipulation.
Advertisements, for centuries, have been a competition to see who is best at manipulating the human psyche - and I find that repugnant and immoral no matter the reason for doing so. Many advertisements today aren't to improve your life, but to fix flaws that don't actually exist but the company needs you to think they do exist so that you'll buy their "solution" to the often non-existent or heavily exaggerated problem. The best companies do this in subtle ways that many people don't realize. Memorable jingles, catchy slogans, smiling, happy, attractive people.
What are you waiting for? Choose happiness [2]
[0] https://www.youtube.com/watch?v=lEtYsKCWnME
