I understand that you would like to see your son on his birthday and perhaps you could discuss this with his step-mother but agree to let him keep the balance of the weekend with his step-mother.
My partner has a 4 year old he had to fight tooth and nail to see on a regular basis as his ex-wife is intent on using her daughter to hurt her ex. REGULAR contact with his new family is as important as the time he spends with you, especially if his step-mother loves him.
You should be thankful that his father loves his son enough to still want to see him regularly and that he has married someone who accepts your son.
My partner has never missed a maintenance payment, never missed a day with his daughter, has cancelled and/or rearranged overseas trips that occur over his weekends - I have too as I love my (soon to be) step daughter as my own. The childs mother constantly harasses us, lies, manipulates her daughter and tells her to lie. Whenever she wants to deny access she just claims the child is "sick". This happened again last night as she wanted her daughter to be present at a New Year party being held at her house. Our plans for the evening with the child had to be cancelled. Same thing happened on her birthday - this year was our turn to have her over on the day. All of a sudden the child is 'sick". Party had to be cancelled, cake thrown away etc.
We have already spent over R50 000 on lawyers fees and next year will be spending more as we cannot plan a single event with a child who loves her father and me as her mother changes her mind whenever she feels like it. This is money we cannot afford and could have been saved towards school fees, music lessons, horse-riding etc. This year we will not accept it - if it means she will be arrested and charged with a criminal offence for violating a court order, so be it. We have tried for 2 and a half years to be "nice". No more.
Many women - perhaps not you - do not think of what is best for their children but what is best for THEM. We have never said a bad word about the mother in front of this child, and no-one else is allowed to in our presence either. The mother on the other hand is always telling her daughter what a terrible person her father is.
All the women posting comments sound like they are just angry with their exes - not all men are creeps. Don't deny your children a healthy relationship with their fathers just because you are angry.
My sisters ex has paid maintenance once in almost 3 years, tells his son he will pick him up and then leaves him waiting, never attends nursery school events etc. Be glad that your husband cares about your son!