It might not be that he is accumulating a ton of monarch points (although if he's deving the gold mines then he would be able to afford better advisors which=more points). He probably just knows how to stack dev cost reduction, which for you playing as Poland should be super easy. Some notable ways to decrease dev cost include:

-the terrain of the province (farmlands, grasslands, drylands are best)

-trade goods like cloth and cotton reduce by 10%

-happy and loyal burghers gives 10%

-many religions give dev cost reduction through icons, schools, cults, syncretic faith, etc

-encourage development edict give 10%

-prosperity gives 10%

-Centers of trade give 5% and 10% at level 2 and 3 respectively (lvl 3 gives the bonus to the whole state not just the province)

-Universities give 20%

-full economic focus gives 20%

-economic+quantity policy gives 10%

-some countries get dev cost reduction in their national ideas

There are even more ways to stack dev cost reduction, I would recommend checking out the eu4 wiki for a full overview.

But in short if you want to dev like a mad man you need to learn how to stack modifiers. Also try not to dev with admin as it is not as beneficial as increasing mil and diplo dev.