A student claiming the AOTC doesn't need to have any earned income; you just need qualified education expenses that haven't been paid with tax-advantaged money (529, tax-free grant or scholarship, etc.). Of course, having tax owed so that a credit can be best used plays a part, but up to $1,000 of the AOTC is refundable, meaning that you can have $0 tax owed and still benefit from $1,000 of the AOTC.
It seems that the biggest issue with your son is that most of his AOTC qualified expenses will be paid by the Texas Prepaid tuition plan, so he doesn't have much to work with to take advantage of the AOTC. Also, if his only taxable income will be around $2,000, most of it investment income, he won't have much tax owed, and a large part of the $1,500 non-refundable part of the AOTC will not be available.