I am so sorry to read that. You've obviously made the right decision, and I know just how you feel. Remember that your vet will have had many people in his surgery breaking down at having their cat or dog put to sleep. They will have seen people in tears many times, and are fully aware that your cat is a family member, and that you are in grief.
Boxtops is right that you need to decide whether you want to bring your boy home with you or if you'd rather the vet dealt with this. There's no right or wrong answer to that. Yes, I'm sure they'll be OK with you just taking him in a box and leaving him, but you might not want to do that when the time comes. If you're a regular at that vets practice, they may well send you a bill through the post rather than having to sort out the finances there and then.
Feeling very sad for you. I know how heartbreaking this can be. x