Both of my DDs had gained up to 18-ish lbs by 6 months and then completely stopped gaining. DD1 still weighed 18 lbs at 18 months, and then started gaining again. DD1 is 16 months old and is only 18 1/2 lbs.
Both kids stopped gaining weight once they started sitting, crawling, walking, running, and climbing. All of their excess calories went towards movement.
Is your LO hitting developmental milestones in a somewhat timely manner? If he is, then I...