I always prompted mine if they didn't say thank you when receiving a gift. I would just say right in front of the gift giver to my daughters, "Say thank you!" And they always did. They've been saying it on their own since they were about 6. Before that, sometimes they remembered, but I often had to remind them.
If he is rebelling and not saying thank you when you prompt him, I agree with the poster who said to hold the gift back until he says it. It might be awkward the first few times, and you can explain to the gift giver what you're doing. If the gift giver doesn't understand and says, "Well, he's just a toddler", then they don't have much understanding for moms trying to teach manners - therefore, it doesn't matter what they think! Enforce it anyway.
It's okay if it sounds mechanical from him at first. You're trying to teach him a habit. As he watches you be thankful in a sincere, gracious way, he'll pick up on it. You can even tell him, as he gets older, to say, "Thank you - you are so thoughtful!", or "Thank you - I have always wanted one of these", etc.
The fact that you are concerned now, leads me to believe that he will NOT grow up never saying thank you. It's a phase, and when he figures out that manners reap good things, he'll catch on quickly.