Thanks for the link. It was indeed a very touching story. Considering his involvement with the pro-democracy group, it is most likely he wasn't allowed to go back to China. Of course this cannot be mentioned in an article published in China.

Yitang never publicly mentioned this but he was likely granted a green card under the Chinese Student Protection Act of 1992. He was also involved with groups that supported democratic movement in China. This explains why he was unable to visit when his father passed away, and the obstacles he faced when he attempted to apply for Chinese visa after becoming a US citizen. He simply would not have been allowed to enter China had he not made the twin prime breakthrough. It was quite fortunate that in the end he was able to visit his mom just a few months before she passed away.