The father-son duo accompanied the Fisher, 32, to her rehearsals, reported US Magazine.
"Isaiah's first band rehearsal...As you can see he & his daddy were totally into it! #naptime #earmuffs #sweetdreams," she captioned the image of her infant son sleeping in the arms of his father, 35, who was also taking a nap.
Fisher and Underwood welcomed their little bundle of joy in February.