My ds was born on the 90th centile and now hovers between 10-20th - he plummeted down the charts in his first few months, and I was really lucky to have a good hv who made me look at the bigger picture as pp said. Ds was happy, developing properly, meeting his milestones etc and therefore the early months just seem to have been him finding the right place on the curve for him. Genetics does play a part - although dh and I are both fairly tall, my ils were tiny, and now ds is not quite on the bottom of the chart for height! Looks like he is taking after them.
I also have to second the reflux - even if he is not spitting up, he seems to have a lot of the other signs of reflux. I would talk to your gp - again, I was very lucky that my gp was extremely understanding when I went in and gave me gaviscon to trial straight away. The difference was incredible. I have also had to cut out dairy - a common issue with reflux babies - and it has made a massive difference to how contented ds is. Also, it can be the case that cutting out dairy can help with weight gain.