He with his mother --(is\are) in Beijing.

1.is
A with/together with/as well as B + V (此时的动词必须和A保持数的一致)
eg:
Mary together with her brothers is going to the USA.
My teachers as well as my little sister are talking about the weather.
2.are
A and B + V.(此时动词常用复数形式)
eg:Mary and Peter are good students.
My brothers and sisters are going to the cinema.