My son is 8 months old now, he is meeting all his milestones but he weighs only 6 kg please help on how to improve his weight.

To help your baby add weight, ensure that you feed him energy dense foods like grains (maize flour,  millet,  sorghum), starchy vegetables like potatoes,  banana,  sweet potatoes,  yams,  nduma),  ensure you fry his food or add oil,  and proteins too especially animal proteins like chicken,  fish, egg, beef and liver.  Feed your baby many small meals through the day.