“Can I claim him if he earned under the $3650?”------> First of all, I guess, to be your dependent as a qualifying child, your child MUST be under age 19 by the end of Dec 31 2010. As he is 19 in 2010, he is no longer your qualifying child; however, as long as his GROSS income is LESS than his personal exemption, $3,650 for 2010, you can claim your son as your dependent as a qualifying relative on 2010 tax return. As you know, you must provide more than 50% of your son’s total support during the tax year of 2010.
“If he did earn more than the $3650, can I file a joint return with him?”--->No; joint return is a US tax filing status that can be used by a married couple , in which income and the deductions for the two taxpayers are combined, MFJ. Your child doesn’t need to file his own tax return UNLESS his gross income exceeds $9,350 as single filer. However, even if your child’s gross income is not more than $9,350 , as said above, he may still need to file a return under certain circumstances; if he is eligible for a tax refund or etc.
“or can I claim his income and then take him as a qualifying child or adult?”------> No; as said above, your child is no longer your qualifying child as long as his Gross income exceeds his personal exemption, $3,650 for 2010.