Saying No to Sharkboy

I fear I am spoiling my child. Yesterday, we went to McDonald's (yes, I've seen SuperSize me, yes, I understand how incredibly gross this food is, but we never eat the fries, I get the Apple Dippers and milk with the Happy Meal, and I personally only eat a junior hamburger, which is the FDA recommended portion of red meat, so get off my back already!)and he got the new toy of the month, "Shark Boy", from some silly sounding movie "Shark Boy and Lava Girl." Jacob knows nothing about this movie, but hey, he got a new action figure, so he was thrilled. Shark Boy and Spiderman had a great time attacking each other in Jacob's car seat. Later that day, we went to a museum downtown with some visiting family members. Jacob was way overtired, seriously needed a nap, and was being downright bratty to his cousins. He was still holding fast to SharkBoy, but when he was looking at one exhibit, he dropped SharkBoy, then left him, forgotten, on the floor. I saw him drop the toy, thought, "There will be hell to pay if we lose that stupid thing," and was promptly distracted and didn't pick it up. We wandered to another exhibit, and I remembered the forgotten toy. I asked Jacob if he had it, and he checked his pockets, then held out his empty hands and shrugged. "Where is it, Momma? I need my Shark Boy." We quickly went back to the previous exhibit, and I asked the docent if anybody had turned in an action figure. A woman standing nearby said, "Oh, I saw that toy on the floor, and a little boy picked it up. Oh, it belongs to you? Sorry, I thought it belonged to the boy who picked it up." Jacob heard this, and let loose a terrific howl, complete with head thrown-back and hot tears streaming down his red face. "I NEEEHEEED my SHAAARK BOY! Who is that boy who took it? I don't know his NAAHHAAME! I can't find him!" and on and on and on. Finally, I said, "Tomorrow we'll go back to McDonalds and get another one, ok?" That calmed him down for about two minutes, until, of course, his cousins did some heinous thing like looking at a rock that made him flip his lid, and he finally fell asleep, exhausted, in the car on the way home. Shark Boy seemed like an insignificant and forgotten matter. Then this morning, almost the first words out of his mouth when he came padding into our room were, "When are we going to McDonald's to get my new Shark Boy, Mommy?" I figured I had to keep my promise, and we went back to McDonald's for the second time in 2 days, and I requested the Shark Boy toy in the Happy Meal. Jacob has not let this toy out of his sight since he ripped it out of that little cardboard box. He is currently killing bad guys with Shark Boy, using our small hand broom as his weapon of choice. But I am wondering if I did the right thing here. After all, it's not like some other kid stole it and ran, or a car ran over it, or something like that. Jacob simply lost it. I could have picked it up at the museum, but technically it was his fault he let it drop and left it for another excited kid to pick up. At what point do you stop replacing toys and start letting the kids learn the lesson of keeping track of their own stuff? At what point do you say "No" to Shark Boy? I did tell Jacob that this was the last time we were going to get a Shark Boy--if he lost this one, it would be gone, that would be it. He seemed to understand, which is possibly why he hasn't let go of the silly toy all morning. Still, giving him a 1 time freebie might still be too much. Any thoughts on how to avoid raising a son who is a complete and total spoiled brat?