My son owned my house for 6 years, and then returned it to me. Is he required to pay capital gains on sale?

Did he buy it from you and then sell it back to you or did you deed it to him and then he deeded it back to you without any payment or consideration?

@pocketsoflearnin let's assume you deeded it to him as a gift and now he deeded it back to you as a gift and there were no other considerations.  

There was no sale, so no capital gains tax. 

However,  your cost basis remains what you originally paid for it, so when  you sell it is when capital gain tax  would be due.  

Who is living in this home?  You wouldn't be eligible for the $250,000 (single) / $500,000 (joint) capital gain exclusion until you lived in it and owned it for two years into the future.  Even if you lived in it for the past 6 years, you did not own it, so not eligible for the exclusion.