You know it's a very common thing for male veileds to do this. Three weeks does seem like a lengthy fast though.
My opinion is that these animals are pretty good at taking care of themselves.
As long as he is alert and drinking, basking, has good color, is parasite free, etc. he's probably fine.
We do tend to be hyper-vigilant with them, which I think is good, but it can cause us a lot of stress.
I will be happy when you post to tell us he is eating again! (surely that day is coming soon)
-Brad
*good comment by Jeweled by the way. If it's too cool (daytime temps) his metabolism will have slowed down considerably. This is the time of year to watch that!
-B.