Flying with 4 month old ALONE

I’m flying with my 4 month old alone in less than 2 days on a 18 hour flight. I bought myself a ticket (2,000$) and my son is considered a lap child. My question is all of the flights have empty rows expect the row I was put in. How do I go about asking to be moved into an empty row so I can put my son in the car seat next to me? I’m flying United if this makes a difference. Do I ask someone at the gate? Would I board the plane at the beginning when they say people with young children can board, or would I wait until everyone boarded in order to be given an extra row? I’m just so confused. My flight leaves in less than two days. TIA