How do I make my 7.5 month old chubbier and make him put on more weight?

He is currently 7.6 kgs and his birth weight was 3.5 kg. He looks very lean... He is on milk and I feed him solids twice daily - in the morning and in the evening. I do not add sugar or salt to his food. Should I add sugar to his cereals to make him chubby? He doesnt feed much on milk too... Just four times during the day...