I really hate to be the odd man out - but I think it is a possibility there's more than meets the eye here...does your Siamese display any other behavior changes, when was the last time he saw the vet? Was everything OK then?
I am one of those ultra-protective sorts when it comes to my cats - so bear with me - but usually any behavior change has a reason. While it isn't always obvious what that reason may be, it is still up to us to determine what it is. Since they can't tell us in the typical way, we have to be real good investigators, and pick up on subtle hints and signs. A possible scenario in this case might be that the other two cats are agressing against the Siamese in the middle of the night. Do they normally get along at other times, do they share a food bowl, do they eat from it happily all at the same time? Something tells me there might be a conflict you aren't aware of between them...and it happens at night.
All that now being said, you could isolate him at night, or if he wants his food nearby, then maybe bring a small bowl of his dry food into your bedroom? (I don't know what your comfort level with that would be - I didn't mind doing it for an elderly cat I had...she wasn't able to move around so much and the other cats got ugly with her when she did)
If all else fails, a vet visit might in be order. Better to be safe than sorry I always say.
Best of luck, and please, let us know how it goes?