How to be welcoming and friendly to an exchange student without being cliché?

Learn Japanese. Also you can get a translation app on your smartphone.

His English will improve greatly as the year goes on. Just talk to him as you would any other friend and ask him to tell you when he doesn’t understand. Don’t get impatient with him. You said you have interests in common; it’s likely his English is better talking about those subjects. As conversation starters, maybe you could watch tv or a movie together and talk about that.

I would begin by NEVER correcting his English. His language skills will improve greatly throughout the school year, but he needs to be comfortable and confident about interacting.

So often, people think it’s so helpful to interrupt continually and “fix” every error. All that does is intimidate and make someone feel awkward. I had that happen to me when I was studying French; I didn’t learn anything except self-consciousness.

If he asks you for vocabulary or grammar, that’s his choice and you can help. But, please let him ask.

I suggest you ask him to join you and your friends when you go soemwhere as a group. That way, you are not focused on him so much, and there are others to share the load of explaing things to him. And with many people along, it is more likely that one might bring up a subject he is enthusiastic about.

Extend a warm welcome and explain that you want them to feel at home and that you are, at heart, a stupid, naïve native and that you will try your best to not be too much of a condescending jerkwad, so, if they could be so wise, to please put up with yourself while you acclimate to that visitors customs and their being there.

The Japanese are a very polite and respecting people. When you meet them for the first time, bow deeply and welcome them to your country/school/home.
Then introduce yourself, and whoever you are with. (Remember to bow accordingly.)
Then determine your ranking by age, the oldest to get the most respect.
Determine how you want to be called: In Japan, you use the last name of the other person unless that person asks you to use their first name. (If your last name is Smith, your parents might be called “Smith-san” and you, being of equal age and disposition, might be called “Smith-kun.”)
An underling, a person of lesser age and importance, bows the deepest. The superiors may only slightly nod or not bow at all. Giving and receiving respect is an honor! Don’t forget it!
If all you do is be very polite and respectful, you will have a wonderful time getting to know and being around this person. Enjoy!

You have some pretty good advice above, and I have nothing to add to it.

I would just like to add that some of the language barriers and the differences in the “mindset” between your culture and that of the exchange students, will be part of the enriching experiences.

My family when growing up, and my church, have a great deal of experience with exchange students from Japan, Scotland, northern Germany and the Scandinavian countries. I only wish I had been a little more familiar with them when I got to experience life with them.

Looking back (and even as I knew it then), it was those differences and communication not always being perfectly spot on,—and even differences in culture, religion and politics, that made the experiences what they were.

The thing that struck me the most, at least back then, was how, in many foreign cultures, there was less value on individualism and a more tighter sense of being like everyone else and not standing out. Yet, here these same individuals were, in a place where they were very different, still contributing to the fabric and overall mindset of the new and diverse groups they associated with here in the U.S. And we got to experience things, such as canoe and rafting trips and road trips and concerts, a little differently just for having them along with us.

I will always remember how some pronunciations could never be mastered perfectly, how some thoughts and concepts could not be conveyed or translated within the perfect frame of reference, but this only helped me to understand foreign cultures all the more. In some ways, people are alike all over the developed world. In other ways, we will always be different. And that’s what makes other cultures more fascinating.

They also contribute more, not less, to group experiences and camaraderie.

This book will greatly help you in helping your new friend to settle in. Foreign to Familiar Understanding Climate Cultures
https://www.amazon.com/Foreign-Familiar-Understanding-Climate-Cultures/dp/1581580223?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-ipad-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=1581580223

In addition make an effort to speak slowly and use shorter words even if it takes you longer to say what you would normally say.

It’s always helpful to ask them to repeat back to you in their own words what they think you said. That can clear up a lot of misunderstandings before they even happen.

If you are out with your friend and he happens to become become sick or injured, remember that his ability to understand English will become a fraction of what it is when he’s well.

Make sure you are also very clear when you go out together or in a group, who pays for what. Your friend may come from a different understanding of how it works.

I would encourage you to take all the advice @snowberry is willing to share. As the mother of adults living and working in other countries, she has a lot if insight regarding language and social differences.
There is plenty of valuable advice above, so I really have nothing to add myself.

Showing interest in having a friendship is bound to be appreciated by him. Cliche is not all bad. The easiest way to learn a new language is through common shared objects, experiences, and activities.