Michael, while you are right, youre not being helpfull.
The optimal weight is different for every bird. While the norm might be 60 grams, it is not unusual for a lovebird to be 40 grams.
The easiest way to feel if your bird is underweight is to feel his chestbone. How much 'meat' does he have on either side of the bone?
You can check the diagram in this topic to compare your bird to:
http://theparrotforum.com/viewtopic.php?f=8&t=1987&p=9252&hilit=healthy+weight#p9251Its a bad idea to feed it 'fat' food. This will just make him fat instead of getting him to a healthy weight. You need to figure out
why he is underweight. Do you feed him enough? Do you feed him the food he needs to have? Is he healthy? maybe he has a worminfection or something that prevents him from digesting the food he eats.
I think it is a good idea to take him to a real avian vet (not a normal vet) for a check up. Its always a good idea to make sure there is no illess that is the cause of the low weight.