Your son was breast feeding till 3 months of age and was gaining weight appropriately on that. Since then his weight gain is only 1 kg in the last 3-4 months. This is very alarming. He should have gained around 2 kgs in this period. Fisrtly it is a little strange that your milk production stopped suddenly but nevertheless you should allow him to suck at the breast that will increase your milk production. And whatever milk you are expressing and giving him, he can take directly from the breast. The more a baby breast feeds, the more is the milk production and vice versa. You will also have to gradually start him on solids like khichdi, daal, mashed banana, porridge etc.
But this still does not explain such poor weight gain. If you were giving improperly diluted formula milk that could be one reason. He was a preterm baby and these babies are more predisposed to infections and other problems. He could have oral thrush
or ulcers which make it difficult for him to feed. I advice you to have him examined by a paediatrician. He/she will check his other growth parameters like length, head circumference and will look for possible local or systemic causes of decreased feeding. In the meantime please continue to breast feed him with gradual addition of solids. You can take galact granules/ satavarex powder with milk to help in increasing your milk production.
Hope that helps.
Wishing you good luck.