As long as he is within the accepted height and weight range for his age I wouldnt worry but if concerned then mention it next time you have him at the Docs.

Mine were relatively small in size and still had that round belly at age 5

Reading books on child development may be of assistance and provide information on weight.

It will happen by about 5 or so.

A GP visit can hopefully address concerns and alleviate any worries.

Speak to your gp about it for advice

If you are concerned, having a chat to a pedestrian might be the way to go. All kids are different and grow at different rates. Some kids can put on a little weight when they are ready to have a growth spurt, others can just put on a little chub when they aren’t being Active enough or have eaten to much bad foods.

I think when they start school around the age 4-5 yrs that is when they start to change you can make an appointment with your local child health nurse if concerned

When he’s eating healthy and has plenty of exercise / running around I wouldn’t be too concerned. My 5yr old has Down Syndrome and LOVES her food. She eats healthy, no junk, no sugary items and eats diary free. Her Pediatrician said he thinks she’s a bit overweight and I could try to give her about 10% less food. However my child doesn’t understand. I’m trying to be moderate in what I offer and offer her healthy food & snacks and when she asks for food all the time try to delay and distract. However it’s not the end of the world when she’s a bit chubby.

Maybe if you are worried ask your health nurse or family GP? When my brother was a kid he would really stock up then one day he would just shoot up in height & thin out. That was how his body worked through his whole growth & he is now 28 and 6ft5! It did take ages for him anyways. If it is a concern definitely ask a doctor. xxx

If you are concerned, maybe book an appointment with a child health nurse and/or pediatrician. They will check your child to see if they are in the healthy weight range for their age, and also check your kids eating habits. Then they can determine if some modifications need to happen.
I know my kid has spent a lot of time indoors the last few months and had some winter chub, despite her very healthy eating choices. So any chub she has put on, is just winter chub. But in the last week, I have noticed her starting to slim back down again, and still not outside much, so must be a growth spurt.
Outside of that, I would look at yours and Dads body type. E.g. My eldest daughter is tall for her age, healthy weight, but it’s mostly torso length, as she has “stocky” legs/butt. But I have always had thick thighs and Dad has a thick butt, so while she can fit a size 4-5 in leg length, she needs size 6 to fit for her thighs and butt & size 6 tops.

Is your son fairly active and runs around a fair amount playing? It may be he is sitting around and not burning off enough energy. If he snacks / grazes instead of eating proper meals you may need to stick to offering fruit even if you have to cook it. I suggest you have a look on google. There is a comprehensive list of foods high in bad carbohydrates that you give you a quick short burst of energy then you feel tired and don’t want to move at all, Good ones fill you up more and for longer. Carbohydrates convert to sugar, some more than others. You also need to consider how much fat/s are in the food he eats. I had to check this out and consult a good dietician. Being too chubby or even worse overweight can cause diabetes and other problems. There is also good and bad fats. Bad fats can lead to high cholestoral which can cause a lot of problems. It can be hard changing diet or even just part of it as an adult. Perhaps you could take him to the Dr. for a checkup. Something else may be picked up that could be causing it.