When did he start moving? Is he really active? I know DD2 gained 400g from 9m-12m and that was exactly when she started crawling. She lost her belly and was also teething so became really picky with food.

Could it be that he is just settling in to his body shape? If you have tall, lean genes it may be that his weight is just settling down into what it should be.

Is he otherwise happy, alert, developing as he should be? Are there any other casues for concern? If he has intolerances he would most likely be grizzly, have tummy issues, skin issues etc.