He sounds like my ds - mine made it all the way down to the 8th from the 92nd! I was really lucky that the HV I saw looked at him and came to the conclusion that he was happy, healthy, thriving, meeting his milestones etc and didn't make a big deal of it. I had to get him weighed more often, I think mainly just to check that there was no indication of any other issue, but he was always absolutely fine. He also had the chubbiest thighs known to mine, despite being titchy!
This always seems to me to be a mummy instinct thing - if your gut is telling you that he is okay, feeding the way you would expect and satisfied between feeds etc, then I wouldn't let the HV push you into anything that you are not comfortable with. I'm sure that you will be able to tell if there is something wrong, so trust yourself.