Here's the thing: your FIL only knows about your plans because you (and/or DH) told him about your plans. Presumably as a conversation topic. And he, as an adult, is absolutely allowed to participate in conversations that are brought up to him. You do not get to just monolog at him and expect him to nod and agree to everything. You just don't, that not how humanity and conversation work.
You don't have to like his opinions, or agree to his ideas, but if you bring up a topic, he is actually allowed to have an opposing idea, and he's allowed to carry on the conversation mentioning his ideas. You can then change the topic if you want, but its really quite unreasonable to expect him to never voice disagreement or a counter idea when you bring something up. Especially not something as extravagant as a 2 week vacation!
So if you and your FIL don't agree on this trip, don't discuss it with him, or anything related to the trip with him (so no lamenting over not being able to afford other things because all your money is going towards the trip, for example). And if he brings it up again, beandip or if he presses simply say "its really not up for discussion" and then beandip.
But don't think you get to just go around talking about this trip to your FIL but that he's not allowed to have an opinion, because that's unreasonable.