There are many ways to promote a t-shirt shop. It's almost like any other business in that you to identify your target customer market and then advertise your site in front of them in as many ways as possible (Search engine marketing, links on relevant websites, directories, press releases, advertising in magazines/newspapers/newsletters, etc).
I did check your source code and found that you use the name of the characters a lot as Meta file keywords you did not use enough of what your site is about and that is selling tee shirts. When people are looking for tee shirts they usually type in a search for keywords associated with the product. I run a cattery directory site for people looking for pedigree cats (yes I do other things besides tee shirts) So if you went to google and typed in "pedigree cats" I would be number one of over 2 million indexed (It is Feline USA.) Now it took me a few years to get there... but I am there and I get a lot of hits because of it. Now I am trying it with my tee shops both wholesale and retail.
Meta tags aren't really as important as they used to be, but he does have t-shirt keywords in his meta tags.
When people are searching for t-shirts, they are usually pretty specific. So they would search for family guy t-shirt or stewie griffen t-shirt.
His TITLE tags and actual content on the page will probably help him more with ranking rather than more t-shirt words in his meta tags. He should also look into a "search engine friendly" package for his shopping cart so the URLs will be much cleaner and easier for search engines to spider all the pages.
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