All kids are different. My daughter didn’t reach 40 lbs until after she turned 5. She’s always been super thin but her doctors have never been worried. Just try to keep him on a healthy diet and limit his sugar. Does he drink a lot of milk? Milk is loaded with sugar.

It looks like his height doesn't match, Im not sure but maybe he didn't get his growth spurt just yet. I wouldn't worry too much about it.

I hate the BMI. It really has to do with genetics alot of them time. Aubrey's doctor wouldn't leave me alone about Aubrey gaining weight because her BMI was low...
But I've always been "underweight" since I was a child.
Her dad is 6'3 and 225 and he's considered "overweight" " based of his BMI but he barely has any fat on him. Got a little bit of a belly but not much.
BMI is absolutely ridiculous. As long as baby ain't gaining weight from sugary food or drinks I wouldn't worry

I think the BMI charts are ridiculous to begin with honestly....but healthy eating & physical activity are both things our ped gives us papers on every well visit. And my kids are slim...my almost 6 yr old is 3’11 & 45lbs & my 2.5yr old is 3’1 & 33lbs.

It is probably because he is in the 50 almost 60 percentile for height but 90 for weight. My kiddo is large too, 90+ percentile for height and weight. We just aim for protein, less carbs (especially unhealthy ones) and lots of fruit and veggies. No juice to drink, but we've never done that anyway. I think they just want to see that height/weight more balanced, but kids grow so fast...just focus on those healthy choices and it will be a-ok. I agree not to focus on it/make it a worry.