That's pretty lame. Honestly, the temptation is real, very real and very tempting. You should definately go to Amsterdam for you, not just for him. It's an awesome city regardless of ganja and prostitutes. Plenty of stuff to do. But if he went with his friends and got high and all the machoism kicks in, surrounded by beautiful scantily clad ladies, things can and do happen. Trust is vital for a relationship, as you say, if he can't trust himself without you there basically telling him not to sleep with other people, then how can you trust him? Amsterdam is a heaven for single guys but not for those in relationships. If you wanna feel secure then go with him and don't let him out of your sight, or find some1 new. if u can go then go.
My advice is if your boyfriend wants you to come just because he can't trust himself to keep his dick in his pants around prostitutes, you need a new boyfriend
Make it so he wont be able to trust YOU. Smoke lots of reefer and do lots of mushies. buy some wooden clogs or something.