Try Land of Nod. They have some prints that look like vintage finds but aren't. They also have bedding in a baseball theme. They are all super cute and fairly priced.

Here's what came up at Pottery Barn Kids in a quick search for "baseball":

http://www.potterybarnkids.com/search/results.html?words=baseball&page=1

A lot is on sale too. The display collection on the second page if your son has his own gear to show off, or you could find vintage items to display as well.

I second the Land of Nod suggestion. Also, you might look at Hobby Lobby for some prints or other decor, if you have one nearby.

Target has some large prints that are awesome. They are team oriented (yankees, red sox, cubs) but have a vintage feel to them.

http://www.target.com/Yankee-Stadium-Framed-23-5-30/dp/B001KP5OS2/ref=sc_pgc_r_3_0_14289341/177-8978197-0642227?ie=UTF8&frombrowse=1

this is the only one i could find online- but stores may have others.

Check out the screen prints at www.sportspropaganda.com -- Chris Speakman has some cool 1940s style designs, but featuring today's players/teams/ballparks. I've ordered prints from him before as gifts, and they're really nice looking!

You could buy a huge frame and lay out vintage baseball cards. Imagine a patchwork from afar, images of baseball "greats" up close. Don't discount Ebay too. You never know what could emerge from someone's basement that is kitschy & cool.
I have an old sign that is a round baseball from the 50's that I purchased from prominent judge's estate. It was from his boyhood room & SO cool!
Headboard made from old bats...
Oh man, would'nt it be cool to find an old scoreboard?!