There are several ways of expressing affection for your loved ones in Japanese. However, it depends on how deep your relationship is. If you have been dating your boyfriend for some time and you think he’s the one for you, you can use the following:
I love you.
Other variations of this are 愛(あい)している (aishiteiru) or 愛(あい)してる (aishiteru).
I like you.
A variation of this is だいすきです (daisuki desu), meaning “I like you very much”.
So choose the right expression for your situation and I wish you all the best!