You can claim him but if your return rejects because he claimed himself you will have to mail your return.  He will have to amend his return.

The amended return won't be ready until late February.

How to amend an Online return

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

thank you so much, but I have new question, because my son not answer for the questions :someone can claim you as a dependent" , he left it blank. How is IRS determined that he claim himself or not?

p/s: if I claimed him as a dependent, it will effect on him refund?

If he claimed himself and  you mail your return and claim him then the IRS might send you both a letter to find out who is suppose to claim him.  It may take a long time to get the letter.  

If he amends his return to say you can claim him it might change his refund if he got any credits like the EIC low income credit.  So he might have to pay some back.