No, I have never had to experience this, but I do have experience giving people bad/disappointing news. First, tell him something complimentary, like how happy you and FI are that he is attending. Next, give him perspective, like how excited you are about the wedding but you have to juggle your wants with the wants of others. Now, break it to him gently, like "I know this means a lot to you but..." . Next, try to offer him a consolation, like he will get the first father-bride dance or escorting someone else down the aisle (grandmother, sister, etc). Lastly, remind him how grateful/excited you are to have him. Its good to practice this aloud so you can get the wording right. I hope this helps!