The term "Senpai" is usually reserved for someone older than you in school or the workplace. It also means that you respect that person, or you aren't familiar with that person who is older than you. I don't know if you should use it with your boyfriend, but maybe you could use "(boyfriend)-chan". It's an endearing term commonly used between partners. I hope this helps :)

No, in any usual/everyday romantic relationship you don´t use that suffix for your boyfriend. It´s generally used as a honorific expression in professional or strictly official contexts. There is no direct tranlsation but it comes close to 'sir' or even in English. In addition to the casual 'chan' you could also just use the more polite 'san' さん as a suffix, in case your relationship is still in it's early stages