Unfortunately, that's the way the law is written… it doesn't matter whether your son was on your Marketplace insurance when he earned his income or not.
According to "Household income," on page 3 of the 2018 Instructions for Form 8962, you have to include
"the modified AGI of each individual whom you claim as a dependent and who is required to file an income tax return because his or her income meets the income tax return filing threshold. Household income does not include the modified AGI of those individuals whom you claim as dependents and who are filing a 2018 return only to claim a refund of withheld income tax or estimated tax."
So if your son was required to file a tax return by federal law, and you claimed him as a dependent on your return, his MAGI has to be included in the computation.
On the other hand, if your son wasn't required to file a return, you wouldn't include his MAGI. I'll attach the TurboTax FAQ "Do I need to file my own taxes if I'm a dependent?" below, just to see if your son was required to file a return.