If my boyfriend (Japanese) said he is busy now and we stopped seeing each other for almost 1.5 months. He said he will contact me when he can find some time, he said sometimes soon.

If you fancy someone you don't stipulate a time when someone can and can't contact you.  Maybe you might stipulate to a woman not to contact you between 9am-5pm because you are at work.  Certainly you wouldn't stipulate not to contact someone over a period of days, weeks or months.

Is he losing interest in me?

Sounds as though he has lost interest in you.  If he does contact you all of a sudden I would be very wary of him, especially if he contacts you shortly before wanting to see you, because that would demonstrate he just wants to use you for sex.

Should I contact him or wait for him to contact me?

You shouldn't contact him, you should ditch him if you can't talk to him when you want to.  There is no point in calling yourself boyfriend and girlfriend.  It is up to him to contact you if he is really interested in you, and then you should be very cautious because it seems funny that he doesn't want you to contact him while he is busy.