As has been touched on, it's not just his weight, but the height gain that's important.
I have had concerns about my ds for years, literally, and kept being told (by paediatrician not gp) to wait and see, as londg as he'ss following his %ile blah blah. I wasn neither happy nor convinced, and he did end up dropping down and down - born near 50th, long time around 25, then 9, now off the scale completely.
he's turning 5 in 3 weeks, and still wears 2 -3 clothes, and is often mistaken for a 2 year old. He is smaller than a lot of 2 year olds, and is only 1 kilo heavier than my 14 month old niece.
At last referred to a growth specialist, who's done an x ray, which reveals he has the bone age of a 2.5 yr old, and has an expected and height of 5'2" or so. We are looking at starting him on growth hormone treatment, but it is unclear why he's failing to grow. He grew normally till he was 2 and since then hasn't growth hardly sice then. Digging out our winter things last week - got his snpw suit which he wore 2 winters ago, and it still fits him - so in 3 years he hasn't grown out of something.
One of the initial Q was - can he still wear clothes that he wore a year ago - even if wieght hasn't changed, height should have.
the calculation of end height using parents height only helpful if no problems or concerns - I have 3 sons, and they are all v. different & clearly ds3 will not end up anything like the same height as ds3.
YOu say in your heart you know it's ok. You haven't mentioned how his height has changed. I knew in my heart it wasn't ok. Well, if you're happy he's growing, then ok, but if you're not happy, or if he isn't growing, I would push for a referral, and not to a paeditrician either - they're basically gp for kids, not specialists in growth problems.
good luck.