In addition to regular taxes, self employment income (often reported on form 1099-Misc) is subject to self employment tax. The self employment tax rate is 15.3% - or approximately $1895 tax on the income amount you mentioned above. After various adjustments such as the deduction for 1/2 of the SE tax due etc, a tax due of $1,800 sounds pretty accurate.
Also, because you claim your son as a dependent, he will not be able to claim the full $12,000 standard deduction. You can read more details about how a standard deduction is calculated for a person who can be claimed as a dependent here: https://www.irs.gov/taxtopics/tc551
If your son continues to do this work in 2019, it would be a good idea to make quarterly estimated payments, as contractors do not have taxes withheld from their paychecks the same way that employees do.
