You should count yourself as being lucky.
My ex married a woman who is 9 years his junior, which is 5 years my junior. However, my girls would say how she would say that "You girls are lucky, you got your mom's looks" and would say little things like that which made my daughters believe that she was jealous of me and insecure. Insecure for looks, and the fact that I was the first wife and I had the first child.
Because of her insecurities, she treats my daughters horribly. Before they got married, the girls said she was nice and great. Which I was all for... then once she said her vows, her mask came off and she showed her true colors. She is constantly yelling at my daughters when they visit... when they lived with my ex, she once picked up her daughter's (my ex's baby daughter) infant carrier and threw it at the wall where my eldest daughter was standing in a fit of rage.
She wouldn't buy them things that they needed... instead she would say "Ask your mom, she's rich." Which I have no idea why she thought that... I am a single income parent... whereas they are two income household WITH bachelor's although they chose not to use it. It's just that instead of spending money on myself, like she often did, I would spend money on my kids.
When her daughter was born, she bought TWO $300 car seats, one for each car. Also would buy expensive shoes for the baby when she couldn't even walk... yet, my daughters wore worn out shoes from Payless.
Now, thankfully, my daughters are living with me. When they go back to visit, their stepmom is still and always mean to them. Their father won't even stick up for them to his wife.
So you ought to be thankful that your ex's new wife is nice to your child and reminds your ex to be a father.
However, I understand on how you need to look good around your ex and his "wife". And you should put every effort into doing so... go work out or at least take walks with your son at a park... right before their arrival, go "doll" yourself up... put on a fantastic outfit and dab some make-up on... then when they leave, get back to comfortable. :o)