BEIRUT — Since disappearing in Syria nearly two years ago, an American therapist has missed the birth of one grandchild, then another.

His family in the United States does not know where he is, who is holding him, whether he has the diabetes medication he needs or even whether he is still alive. They have not spoken to him since he told them he was making a quick trip to Damascus from Lebanon to visit relatives in February 2017, packing little more than pajamas in his overnight bag.

Majd Kamalmaz, now 61, had been in Damascus for less than a day when he was stopped at a government checkpoint, according to the taxi driver who had been driving him around the Syrian capital. That was the last time anyone has heard from him.

Like some other Westerners with relatives who have been detained or held hostage over the course of Syria’s long and bloody civil war, Mr. Kamalmaz’s family chose at first not to speak publicly about his disappearance as they tried to push for his release.