Is it a bad idea to shower together with my boyfriend if he has foot fungus? Will it spread to me? Are there any precautions I should take?

It's worth it. Even if it wasn't, the superficial fungi are all over the environment. A person who has a visible infection shouldn't be regarded as presenting an infection hazard. I trust he's getting treated -- tolnaftate is a good, easy-to-get remedy for most tinea pedis infections.