You might consider giving them frozen bottles to lie against instead of just fans. Also, don't blow the fans directly on them--all that wind can actually stress them out more.
Try to entice them to eat by providing yummier food, like rolled oats and sunflower seeds. If they seem to like it, try mixing it in with their regular food. Or, if those aren't quite enough to get them going, try fresh vegetables or fruits.
What are you feeding them? If he was still growing, I'd say you might still have a chance of getting him to grow up to the weight, but since he's over a year (3 at that!), any weight you add isn't likely to be structural. Make sure that if you're feeding pellets that the protein content isn't too high (16% minimum crude protein is usually the best, and should be that high for any growing Flem).
Flemish are also easily stunted by small cages. I knew a buck that only got to 12 lbs because his cage was only 30x30. Flemish need a minimum of 36x36, and that's with plenty of time in an exercise pen. I've seen recommendations as large as 3'x5' per rabbit.
It's also possible that they are from a smaller line of Flemish. Does he have any kits/siblings that you can compare to? You might do a test breeding with him and another Flemish to see if you can get his kits up to a good weight by how you feed them. If his kits grow big, then he may have just been stunted. If not, then they're not likely to produce high quality kits.
