It's definitely not because of the meta tags. Google doesn't use the keyword tage or the description tag. (Feel free to show your client this so he doesn't think he knows it all).
What may be causing the issue is:
Changed URLs. Did you change the URLs the home page or internal pages? If so, did you do a 301 redirect from the old pages to the new pages? If not the new URLs will be considered new pages and start from scratch with PR, etc. If you want Google to associate the old pages with the new ones do a 301 redirect for every page with a new URL.
Changed site structure. Did you change how the site is structured? Did you create a hierarchal system of pages with the most important ones being towards the top? To optimize your internal linking and PR channeling your site structure must be done correctly.
Loss of semantic markup. HTML is an important indicator about the importance of page content and relevancy of page content. If the new HTML doesn't give the search engines clues about the content then even if the mark up is valid it isn't SE optimized.
There could be plenty of other reasons why this has happened that I didn't list here. It would take too long to do. It's also possible that the change is temporary while Google sorts out the new site. If you don't know much about SEO I highly recommend reading up on how it works as it goes way beyond valid HTML and CSS.