I'd check portion sizes first, as it's easy to over feed children if they havent developed a natural stop when they're full.
A portion is just one fist size amount, that's the size of their fist, not a grownups, which is actually not a lot!
I used to be very reassuring and accepting when posters asking about their childs size/ eating, especially when they said their child was tall and therefore in proportion. And I am still supportive, just not quite so quick to assume that parents have such an accurate sense of body shape for a child.
The change is because I met someone in real life recently who said the same, and I thought fine, sounds reasonable, her son is about a foot taller than mine (& mine is 91 percentile), and just a completely different shape to mine and the other kids I know. But he's healthy, active and strong, not to mention just beautiful & good natured :)
I didn't really think about it until one day I saw him with his top off and was quite taken aback to see him - he was properly chunky, and big, with a thick layer of fat all over. Kind of half a big toddler body crossed with a bulky rugby playing grown up.
So not fat in the way that you / we/ people tend to imagine a fat child to be, with a big belly or a waddle, getting breathless etc, but really, way too heavy and too much fat for a child. They aren't supposed to have any of that fat layer on them. Young kids are naturally a very different shape from adults, or toddlers. They are skinny, rangy, with visible ribs & spine. On an adult that would be too slim, but on a young child, it's what they're supposed to be, allowing for some differences due to body type etc.
I'm afraid the child I'm talking about is very overweight, no matter what his parents think, and to make matters a little more awkward, he routinely demands snacks off my son and tends to eat all the snacks ive put in DS bag for after school clubs, even when I started to put in extra so ds could share. Argh, so tricky, poor kid, and poor parents too.
That's going to be so hard to get back from, as now he's a bit older than your son (a couple of years), it's harder to control food going in & help change food habits without disrupting their relationship with food.
If your son does turn out to be over weight, I think it's a good thing you've seen it while he's still little and you can stabilise weight and he'll grow into it naturally.
It's so hard as theres so much shame around being overweight. It's become a moral judgement and an acceptable excuse to treat people badly, make them feel small. And that's a large part of the problem, it's such a terrible thing to be, people don't want to believe it or admit it, and then it makes it even harder to deal with it.
So I want to make it clear there's NO value judgements from me happening here, but an observation about how hard it is to recognise, and then deal with in a practical, low key positive way.
Good luck whatever turns out to be the case