What a great gift! Your son is fine as he sold a gift that was given to him. The question is more around the gift. If the $20,000 came from 2 people then it falls under the gift tax limit, but if the gift was just from 1 person that person will need to simply fill out form 709 https://www.irs.gov/pub/irs-pdf/f709.pdf
I hope that this is helpful.