Aggressive pets

Cheyenne • Momma of two angel babies 2/1/18 👼🏻 2/3/19 & a sweet girl 12/15/19 👶🏼

So we have 2 chihuahuas and they have never been aggressive or bark/growl. Lately though the younger one (he’s 2 1/2) has been getting more aggressive toward my SO and I don’t know what to do about it. This dog has never been around abusive men, or any kind of abusive person for that matter. They used to live with 2 young kids (4 and 6) so I don’t think the problem is him being agitated or anything. Any suggestions??