This guy is an idiot. He bought the ticket for his 18 year old son not his baby. You can't switch passengers in and out of seats on a  plane, that will set a dangerous precedent. 
Most of the complaints with airline are due to unruly passengers not following the rules.
The mother 2 weeks ago caused an issue when she wanted to  take her stroller to her seat. Can't do that either. Rules people, follow the rules.