I think you've been given pretty poor advice by your doctor.
First, dropping one centile is no reason to be concerned, it is commonplace, because the lines are based on an average, and individuals vary. Some will grow faster, some slower. All normal and healthy. Usually crossing two centiles would be the point to worry.
Secondly, there's nothing in your post that would give a reason to assume your milk supply is low at all, never mind in jeopardy - it makes me angry that you were told that. At fifteen weeks of feeding a baby enough to grow on the 25th line, you probably have a good, established supply.
Does he seem alert? Happy? Pooing and peeing? These are the signs of a well fed baby - he would have let you know if he was hungry, by being listless, sleeping lots, not pooing, not to mention crying.
Third, giving a bottle of expressed milk will give no information whatsoever about how much he takes in a feed. He could take less, as he might like it less than from mum, or more because bottles are easy to get milk from, you can't tell. Nor does what you can express indicate how much milk is there - some ebfing mums can't express a drop, it's totally different to a baby feeding. Bad advice, which shows a lack of knowledge about bfing. (the same goes for giving ebm after a feed - even a full baby will often take some from a bottle, it's a reflex to having a teat in their mouth)
If you are worried, (because your not very well informed doctor has made you worried 
) all you need to do is offer feeds more regularly. If he wants more, your supply will soon adjust, without drugs, herbs, anything. He may not want more though.
If he is refluxy, formula can make it worse, as the proteins are harder to digest, and usually things like keeping him upright after a feed are introduced before trying any medicines.
What would you have said about his growth/development/eating before you saw this doctor?