That’s not a „clear source“.
It’s not clear if it was „his“ vote, or just the kernel of his mum that he cast for her.
It's not definitive, but the
DMR article about Gillibrand's son says nothing about it being her kernel, and the wording (at least to me) implies that it was his own:
Before the presidential hopeful spoke at The Des Moines Register Political Soapbox on Saturday, she stopped by WHO-TV's Cast Your Kernel contest, a casual way fairgoers show support for the candidate of their choice by placing a kernel of corn in a jar labeled with the candidate's name.
Henry, age 11, wavered over to Warren's jar and teasingly contemplated casting a vote for the U.S. senator from Massachusetts.
Gillibrand gently moved Henry's hand up toward her own jar, where he eventually dropped his kernel and said, "She's pretty good. I'll vote for her."