Did the dr say anything else? Did he look dehydrated? anything clinical or is it just the weight? Seeing/pooing/behaviour all fine? Is he actually losing weight or just putting it on more slowly than average?
Generally speaking, dr.s, even m/w and gp have very little bf experience. They have no time or knowledge to actually help. The extent of most bf advice is "top up with formula" which usually sets up a slide into total ff.
What do your instincts say? Do you think he's starving? Do you think there's something wrong?
Babies will nearly always take formula after a breastfeed. It's a bit like us having a full roast dinner- you think you't not hungry then someone offers you a wafer thin mint- you squeeze it in because it tastes nice.
FWIw mine slipped from 50th to 0.4th in about the same time. I had a very good hv who never even mentioned formula- gave me lots of tips and tricks about increasing bf instead.
I was referred to the GP who also said top-ups may be a quick fix with the numbers on the scale, but if there is anything actually wrong, that weight gain may mask it and delay investigation/treatment. So we were referred straight to paeds for bloods and examination. First question was "are you worried, or is it the hv/gp?". I wasn't worried over much because she was so well and happy in herself. Bloods were fine, measurements were fine, paed happy, carry on bf.
Google "catching down". Some babies seem to take a few weeks to find their line. Mine stayed happily on 0.4 and is still thereabouts 10 years later.
Kellymom is your friend. Lots of very sensible advice :) Try and get a rl consultation with la leche or a bf specialist- this is what your GP should have done rather than take the easy way of top up with formula. At the very least they should have tried to help with the bf- and if they were genuinely worried about the weight you should have a paed referral.