Buying a ticket for a minor separate from adult (traveling together)

I hope someone can help me with this.
I am taking my son to spend some time with his grandparents. I need to buy a round trip ticket for myself going and coming back the same day, another ticket for myself going and coming back the same day few weeks later. I also need to buy a ticket for my son going there one week and coming back few weeks later.

Any way I can do it online?