16mth not gaining weight and ht?

At 16 months those measurements do sit on the lower end, so it is worth going through whether the issue is appetite, food variety, or something else entirely before deciding next steps. Some kids are just naturally smaller and track steadily along their own curve, while others genuinely need a bit more investigation into absorption or underlying health factors. I would bring him to the PD with his health booklet so they can plot the growth trend over time, because a single reading tells less than the pattern across several months. If the PD feels it warrants it, they may refer you to KKH or a dietitian for more targeted advice.