If you son is excited by science, he can have a science-based curriculum and still learn other subjects. He can read and write about scientific topics. He can do experiments and write reports on them. A large part of scientific research is recording the data and analyzing it.
Math is also a huge part of science. He needs some experiments that require taking measurements and then using those numbers. Find averages. Find the largest or smallest. Find fractions of things.
Do you know anyone who works in science? A chemist, medical professional, etc? Maybe they can help. If he is interested in biology, try the local conservation center. Ours is great, even if it is run by the state. They are very homeschool friendly and let us check out the kits used in schools.
Thanks for the words of wisdom from yourself and taxchick. I guess I get down, because my husband believes that our son needs to have things a bit more structured. It's when I try to structure things that we have these problems. There is an astronomy lesson he was supposed to do yesterday, but didn't get around to because of the other topics. I'll just lift any misspellings from that.