You cannot take the SAT and SAT Subject tests on the same day. You can, however, take up to three SAT Subject tests on the same day.
There are a few restrictions on which subject tests you can take in one day: language-with-listening subject tests are only offered in November and you can only take one language-with-listening subject test per test date; you have to take either Biology E or M on the same date (you can't take it twice). In general, I'd advise against taking three subject tests on the same day, unless time is really running out for your son to sign up for the SAT Subject Tests; in my experience, anything more than 2 tests in one day will result in poor results on all the tests.
In terms of which order to take the tests, I'd advise your son to take the SAT Subject tests as close to when he's done with a particular subject in school as possible (for instance, take the Physics Subject Test after he's taken Physics). A good schedule for SATs and SAT Subject Tests (and one that I followed myself) is to take SAT Subject Tests in May (towards the end of most AP or advanced classes) and the SAT in June.