Upvote:3
On entry, the 90/180 rule does not matter (yet), one concern is that he must in principle satisfy a number of other requirements (valid purpose, financial means, etc.) which are not relevant for an EU citizen. The interview typically isn't very extensive but it's a concern he would not have as an EU citizen.
During his stay, as an EU citizen he would enjoy a number of rights that cannot possibly be denied because he once entered on a US passport. But, in practice, he might very well need his national ID card or passport at some point (say if he would decide to reside in a country like Germany or the Netherlands, where everybody needs to register with the authorities). But he could still go to Poland and obtain one after entering on a US passport.
On exit, if he stayed longer than 90 days, then proving his EU citizenship becomes relevant, as a US citizen could still be fined on the spot for his overstay. A passport or national ID card is obviously the best way to do that.