Plum syrup juice instead of red shiso juice

I have a little lovable nephew, 7 year-old,  who likes my mother's home made red shiso juice.

Here is one episode.

When he visited my house to stay over some nights last year, I welcomed him with my home made ume juice. At that time words popped out of his mouth was unforgettable.
He said "This is not tasty, don't you have red shiso juice in your house?"

But I had never made red shiso juice and so of course I had no red shiso juice in my house.
I said "Sorry not in my house". Listening to my word, he was so disappointed.

I felt regretful that I could not make him happy because of no red shiso juice in my house.
This is why this year I've started to make red shiso juice.
I asked my friend who loved to cook. She introduced me one recipe, saying "This recipe is the best".

The website she recommended is ちいママさんちのしそジュースシロップ .

In fact her word didn't betray my expectation. This recipe is easy-to-follow, besides tasty.
If you understand the instruction that the cook said on this website, you cannot help smiling.
The Japanese itself is cute, especially I like her child's comments.

Anyway I successfully made my first red shiso juice thanks to this wonderful recipe and enjoyed tasting every day every night... then it had gone easily.

Since I felt confident I could make red shiso juice any time, all I have to do was expecting my nephew's visit.

A few days ago, my nephew rang me "I would like to visit your house when my summer vacation starts." He said he planed to come here next week.
So, I went to a supermarket in high glee last Friday to buy red shiso, but.... to my surprise red shiso was sold nowhere.

No, no, no!

It seemed that the best season of red shiso was over. I didn't know that.

What should I do to satisfy him?

Suddenly I hit upon making plum juice, though I had never tried.

1) Because I found many plums in the super market
2) Plus I remembered one recipe which I had came across on a magazine the other day

Since I didn't remember the recipe in detail, today I went to a book shop to browse.

Here is the recipe (though I changed some parts because of some reasons)

<Ingredients>
Approx 500g plum(大石早生）
130g white sugar

 <How to cook>
1. Cut cleanly washed plums in halves, put them into a pod, and add over the plums.

2. Mix them with a hand.

3. Simmer over low heat  (I don't remember what time it took but waited until colour of the plums turned to be like below.)

The taste was just like sour peach juice. 

A little bit sour than I had imagined before cooking, this may be because I didn't use fully riped plums that was recommended on the magazine. (Unfortunately, I couldn't find riped plums at the super market.)

I have totally no idea this can be my nephew's taste, but I am looking forward to welcoming him next week with this juice.