He’s just hungry or feeling a need for comfort. I breastfed both my kids and I have to be careful in public holding either one of them. Even my long since weaned 3 y/o daughter if feeling particularly nervous will try to grab a boob. The 15 month old will just dive in. And, crap, my daughter as a baby would basically start lifting my shirt. Breastfeeding isn’t for the overly modest.