full kanji for sushi = 寿司

Maybe the other side of the mug has the second character?

This pose was very deliberate. Is he flashing he likes sushi or Asian men?

Twitterverse seems to be creeped out by this photo.

Another reading for this kanji combination:

寿司 • (Hisashi) a male given name

This may be most likely: 寿 = shinjitai = congratulations or longevity

Congratulations on his new job?

Shinjitai, 寿, is often written on gift envelopes in Japan.

Maybe someone familiar with the Japanese for "congratulations" gave him the coffee mug as a gift.

司 Can mean "boss" or "manager"

If I show this mug to my husband, he will say, "That is shinjitai, congratulations."