Registration in Spain and tax implication n visa (Barcelona)

Hi all I am not sure this is the right place to post but couldn't find a more suitable one. I am doing some research as my bf and I are moving to Spain next year, I am US citizen n he is Dutch (we have officially been living together for a year)...I read this website Protected content and it states that he needs to register at the central register for foreigner and then I can use that to request my spouse/family member residency card but this to me sounds like he would be officially registering as a resident of Spain which would have tax implications for him and we are not sure what we are going to do in that regard yet there seem to be lots of arguments to support registering and other against...does anyone have any thoughts on the matter. Thanks in advance