I have an question, my boyfriend is Slovenian (I'm Dutch) and he is working and living in Belgium. He is fully registered and paying Belgium taxes etc. Now we got the opportunity to live in the Netherlands. Does anyone know if he can register in the Netherlands if he is working in Belgium while being Slovenian?
There is no reason it should be a problem. He will not qualify as a worker but EU citizens also have a right to reside in other EU countries if they have sufficient income and health insurance (and his Belgian job will take care of that) so he will be covered by the free movement directive even if he does not have a job in the Netherlands. As a cross-border worker, he will be entitled to health insurance coverage through the Belgian statutory health insurance system even he does not reside in Belgium, which should satisfy the Dutch health insurance requirement.
In practice, as he is an EU citizen, he will just have to present a rental contract or some such (if he is going to live with you, you will have to sign a special form and give a copy of your ID like you would for a regular Dutch citizen or resident) and an ID. He should not be asked for a verblijfsvergunning or to prove that he is in fact entitled to reside in the Netherlands (i.e. if he would not qualify based on his income/health insurance status, he could be asked to leave as far as EU law is concerned but the Netherlands is not actively enforcing this restriction AFAIK).
Tax implications can be complex however and I imagine dealing with Belgian bureaucracy might be too.
He can register in the Netherlands. There are many grensarbeiders, including foreigners, who live in the Netherlands and work in Belgium or vice versa. However, I recommend he discusses his exact situation with an accountant who is knowledgeable about NL/BE cross border tax rules, to understand what tax he will pay and avoid any nasty surprises.