Boyfriend booked a holiday with the "lads" over my birthday

Okay, so I have been with my boyfriend for 9 months now and we are going strong. But recently, I found out that him and his friends have booked a "lads" holiday to Zante. He is the only one of his friends who isn't single and I am worried that he may sleep with someone out there.

On top of this, he's leaving the day before my birthday as that was the cheapest time to go :/

What do I say to him? He has every right to go but he'll be missing my 18th birthday to do so :/
Boyfriend booked a holiday with the "lads" over my birthday
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