What do I message to a person whom I like..... to start a meaningful conversation (without showing that I'm interested in him)

The funny part is that you ARE interested in hm, but you don’t want to show it. Maybe when you turn 16, you will understand that coyness doesn’t get you very far, but that honesty does.

Did you ever consider that he is just as shy as you are?

Here’s my suggestion.
1. Find a word in the dictionary – something unusual.
2. Ask him if he can help you figure out what it means.

If he’s as smart as you say, he will either know, or he’ll engage in conversation with you.

Say that you are curious about something and his name came up as a possible person to ask for a reasonable answer.

Send a photo of something you think he’ll like, for example, a dog photo. Use the photo to start a conversation, like “we went to the animal shelter today” or “This is my aunt’s dog.” etc.

“I’ve heard you feed stray dogs. I’m interested in hearing more about it.”