Baby showers for boys can be really fun, and the themes available are diverse. The style of your event has an impact on the themes. Our company run a bridal and baby shower planning company, and we stock over 70 baby shower themes. Here's some options.
-Retro themes. Themes like a "little red wagon" are fun in that you can feature the bright red, white, and yellow colors of the traditional Radio Flyer wagon. Nothing says little boy like a red wagon, and the wagon theme can be taken in a lot of directions. There are miniature wagons (I think they are size # 5) from Radio Flyer that are perfect for centerpieces, and invitations, thank you cards, etc., are available featuring the wagons.
-Jungle / Animal themes. This theme is often used when people are decorating the nursery with an animal theme. Three of the most popular are a lot of options including 'Monkeying Around', 'King of the Jungle', and 'Wild Safari." These all feature fun variations of the animals, but embrace very different color pallets. The jungle theme can be carried throughout the venue for your event including palm trees, groups of bananas, etc.
-Traditional / Religious themes. Precious Moments, Noah's Ark (or Whimsical Ark), Blessed Boy, and other themes are perfect for this type of event. The inspiration and decorations can be spiritually inspired; very good if you are holding the event at a church as well.
-Sea / Nautical / Whale / Turtle / Ocean themes. There's a great new tableware and baby shower decorating pattern called Ocean Preppy. You can use the colors of the ocean (greens & blues), and host a "whale" of an event.
-Owl / Woodland Animals. This pattern is hot right now. Perfect for an outdoor event, you can take inspiration and use lots of branches, logs, moss, and such. There's one pattern called "Happi Tree" which features a momma and baby owl on a branch, and small critters like a hedgehog. The baby related puns for an owl inspired event are plentiful. "Whoo loves you baby?" "You'll never believe Hooo is arriving soon. Etc.
- Candy / Cupcake showers. These are fantastic if you (and your guests) have a sweet tooth. Set-up a cupcake bar and have guests decorate / frost their own cupcakes. You can also set-up a wonderful candy favor bar. Fill glass vases or large jars with candy in the colors that coordinate with your decorations, put spoons and tongs in the containers, and set out favor treat boxes & bags. Let your guests create their own favors.
Check out the site referenced below for over 70 different decorating themes.
Hope this helps.