They may be a weird shape to him, make odd noises or smell odd. There are a number of things that could make them upsetting to a dog that has little experience with them. For odd items that I want to get my dog used to, I play the "touch the thing" game. I teach him a touch command (to touch something with his nose or paw when I tell him touch) and then he gets cues to touch the (insert: shovel, cooler, broom, etc.).
Especially for a puppy, building confidence to experience and habituate to new things is important to socialization and getting them in the habit of approaching new things makes each successive one easier. If you play the game with him, you'll notice after a few objects that he is more confident and excited to explore new things because they make positive stuff happen (a treat or a game).