Can I check taxes 2 different ways to see which is better without submitting them?

Yes, you can fill out information in a TurboTax return to see how it would differ from your regular return. Payment is not required until the return is ready to file, so people do mock up returns to see how changes would impact tax liability.   If he has some income, he can still file a return but will select in the My Info section that he can be claimed on your return and if he will be claimed on your return.  This will impact his standard deduction. 

If he is a student and temporarily lives at school but your home is his permanent address, he can be considered living with you for that time. 

This TurboTax Help article has additional information about claiming a dependent. 

  if he's not in school and over 19 or in school and over 24 you can not claim him as a qualifying child.  for you to claim him as a qualifying relative there are 3 tests.  Your post indicates he meets 2 of the 3 tests. the third is that his gross income must be under $4400.  if he qualifies to be claimed by you, regardless of whether you do or don't he must indicate on his return if he is required to file, that he CAN be claimed as a dependent on another's return.