LittleOneMum, please don't worry, I agree with everyone else who has posted. My DS1, nearly 5, has always been long and skinny since birth. He was the Peter Crouch of babies LOL! He has stayed like it. He is on the 99th centile for height, and hovers between 50th and 75th centile for weight, and has done since he was about 6 months old. He literally looks like a famine victim when he is naked, and I'm sure if most people saw him with no clothes on they'd be a bit .
But my lovely HV has never been worried about him, knowing how healthy and full of energy he is. She took one look at me (I'm naturally skinny myself, horrible stick arms and legs - yuck!), and also met my (slim) DH once, and said "It's no surprise he's built like that, he's obviously going to take after you two". None of this nonsense about getting weighed every fortnight. All she ever said was "As he's so tall, he can take a bit more weight, so don't worry if you want to give him an extra biscuit or two if he wants it!" Totally sensible lady.
You can't force them to eat more if they don't want it. My DS1 goes through phases where he's starving all the time and will eat loads, and then weeks where he's less hungry. But he's obviously getting enough calories as he's healthy and growing UPWARDS at a rate of knots. (he is by far the tallest in his class at school - 18 months ahead of average height of his age group). The way I see it, if he wasn't getting enough calories he would't be growing upwards as quickly as he is.
DS2 at 2.5 is only on the 25th for weight, and about average for height I thnk, but because he's not outrageously tall, he doesn't look too thin.
You don't say much about his diet, but as other posters have said, as long as you're not thinking of "healthy food" in terms of what an adult would think of as "healthy" i.e. low-fat, then you'll be fine. A growing child who is not overweight should have full-fat everything, loads of dairy, bread & butter as snacks not just fruit & veg sticks or raisins type of snacks (I gave biscuits a lot of the time if kids still hungry after healthy main meal and yoghurt for dessert - as long as they got their fruit & veg through the day I didn't give a ff about the odd biscuit, but others might disagree!). I give mine supper just before they go to bed. Usually toast or bread and butter. I'm surprised they are hungry enough to eat it seeing they usually eat a good main meal a couple of hours before, but they do!
Fruit smoothies made with yoghurt etc etc.
It's difficult if your child just has a fast metabolism, which I think DS1 has. He stays packed lunch and can easily polish off: ham sandwich on wholemeal, a yoghurt, some cheese cubes, some strawberries, and a gingerbread man. Other kids eat a LOT less than that in their packed lunch and aren't skinny - I think it's just the way they are.
Do not worry! I wholeheartedly agree with Peachy, people's idea of the "norm" has changed with regard to weight, now that there are so many overweight people about. I have in the past been right hacked off with overweight (and I do mean overweight) acquaintances feeling it within their rights to tell me that I'm too skinny and "need some meat on my bones", when I eat normal amounts of food and am perfectly in the middle of the healthy weight range on a height/weight chart, and they clearly are not (yet I wasn't so rude to point that fact out to them - maybe I should!).