my son filed return with statement that no one can claim him as a dependent, his return was rejected due to me claiming him as a dependent. what do we do.

Assuming your son was correctly claimed as a dependent on your tax return, your son needs to change his answers to say that yes he can be claimed as a dependent an yes he was claimed as dependent.

If he e-file was rejected, he can simply make those changes and re-submit his return for e-filing.

If he is correct that you should not have claimed him as a dependent, then he will have to paper file his return, He can no longer e-file.  In the meantime, you need to file an amended return, unclaiming him.  He doe not need to wait for you to do so. but can mail his return now.  It will take longer for him to get his refund, as paper returns are not currently being processed at the IRS.

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html