My son filed his taxes, but made less than $4,050. Can I claim him as a dependent on mine? On his taxes, I do not believe he indicated that someone else could claim him.

Depends on if he meets other qualifying requirements whether you can take and exemption for him.  If he took his own exemption, then he would need to amend his taxes, if you are qualified to take his exemption. 

https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...