Question: My son is about to go to a junior college in San Francisco. He will do this because he has no idea what he wants to major in. If he spends one year at the junior college, can he transfer his credits to another college or will he lose them all if he doesn't finish two years?
Your son CAN transfer credits from a two-year college to a four-year college, even if he only completes one year (or, for that matter, just one SEMESTER).
Note, however, that--depending on which four-year school he attends and the major he chooses--some of his credits may not transfer. Those that almost always DO transfer are in "core" subjects such as English, history, math, science, and so on.
Credits that might NOT transfer could be in areas that the four-year college doesn't offer and may consider vocational (auto mechanics, travel management, cooking, etc.) but it really depends on the four-year school. Those institutions that have programs in such areas may accept the credits, even if your son will major in an unrelated field.
Other credits that probably won't transfer are in remedial classes. For instance, if your son had to take a basic math course (comparable to a high school class), a reading course, an English-as-a-second-language course, etc. then it's possible that those credits from the two-year school will excluded by the four-year school.
Also, if he's interested in University of California schools, he may have to wait until he is a junior to transfer in. (But that doesn't necessarily mean that he has to spend two full years at his JC. He could attend a different four-year school in the meantime.)
Bottom line: It's likely that all or most of your son's credits WILL transfer to a four-year college, even if he doesn't graduate from the two-year college, but you do need to check this out on a school-by-school basis because you are likely to find inconsistencies.