When does the holding period of property transferred from an irrevocable trust to the sole beneficiary start? Is it when the trust acquired the property?

My son was the sole beneficiary of an irrevocable trust settled by his grandmother, and I was the trustee.  Pursuant to the trust document, I transferred a condominium and all other assets held by the trust to my son when he turned 30 and put the title to the condominium in his name.  He has occupied the condominium since June 2016 and may want to sell it.  Did the 5-year holding period for Section 121 start when the trust acquired the property or when it was put into his name?