In 2x14 Broken, Chimney calls Tommy for help when there's no water to put out a fire. As the plane approaches, Bobby, Hen, and Buck are all shown reacting to it, before the fire is extinguished, saving Eddie and a young boy.
In 2x18, the son of the man who burned down his restaurant shortly after Bobby joined the 118 is responsible for the ladder truck crushing Buck's leg.
As he looks at Buck, he notes that Buck is new, as he is not someone who worked with the 118 when his dad was arrested. Buck took Tommy's place at the firehouse after Tommy transferred to the 127.
(Thanks to @xofemeraldstars for spotting this one.)
In 3x16 The One That Got Away, Buck is afraid of being left behind by everyone else and ending up alone, like Red did. Eddie tries to make him feel better but the conversation ends up making Buck feel worse after Hen and Chimney reveal that the closest thing to contact they have with anyone who left the 118 is Chimney asking Tommy for a favor.
This is (possibly) the last mention of Tommy until season seven.