Firstly, even if you amend, he won't be able to e-file, since the amending process is so much slower, that the e-file database won't be updated before the deadline.
Secondly, if he can be claimed as a dependent (meets the qualifications) then he must answer yes to the question "Can you be claimed as a dependent by another taxpayer" even if he does not want to be. See this,
https://ttlc.intuit.com/community/dependents/help/when-do-i-have-to-answer-yes-to-being-claimed-as-a...
