Two thoughts.
1. Are you feeding him enough? You said 1 to 2 weeks...not sure how big the rats are, but there's a chance it's not enough especially if it's something that's too small and it's every 2 weeks. Just doublecheck that.
2. Star gazing. Is he possibly sick? Star gazing is one of the signs of respiratory infections. Check for the other signs (mucus around the mouth, clicking or wheezing when breathing).
Could be something else, but that's what came to my mind when I read your post.