Greetings of the day
Unfortunately your son cannot attempt the MHT CET exam as he doesn't satisfy the eligibility criteria for the same. It's clearly said that the applicant has to have Indian nationality. That means US residents cannot apply for the exam.
https://engineering.careers360.com/articles/mht-cet-eligibility-criteria
Go through the above link for detailed eligibility criterias for the MHT CET exam.
Hope this helps!!
All the best for your future endeavours
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