From A Wiki of Ice and Fire
A Clash of Kings (appendix)|
A Storm of Swords (mentioned)
A Feast for Crows (appendix)
Quarterly: a silver-grey castle on blue beneath a red bend sinister, three green mistletoes on gold bordered in green|
(Quarterly, first and fourth azure, a bridge cendrée, at either end a tower of the same, all debruised by a bend sinister gules (for Walder Rivers), second and third or, three mistletoes within a bordure vert (for Charlton))
Like his father, he also has a personal blazon.