How did you "request" the weekend? If you are in the habit of just "inform[ing] his father...that [you] would be taking him that weekend" then I might see where the father wouldn't necessarily be so accommodating. Maybe you inadvertantly treat it as your RIGHT when you talk to him, and it comes off as a demand.
Otherwise, I think that he should have known that that time surrounds your birthday--maybe a bigger heads up from you, months in advance--and he should have marked the calendar for likely time with you and planned for the child to be with you, even without your formal request. When my SS was younger, I always knew where birthdays and holidays feel on his visitation calendar, and that sometimes affected change for our weekends.
I say ASK HIM, explaining your birthday wishes and that you requested time off for this. Do NOT ask him what their plans are and try to determine if they are important enough to trump your birthday. That is none of your business. Sorry.