I'm open to any suggestions on how I should fill my day with him.
given i was only there for two days, this wasnt going to turn into some ongoing work, i think giving me some hands-on and engaging thing to do - despite it being totally independent to the project they were working on - was probably a good idea. from memory i found it pretty interesting.
When I had a corporate job, I did an hour of job shadowing here and there for specific purposes. When in another department, I just watched as they did typical work so I could better understand what they did in a department that supported ours. In my own department, there was more explaining because I was shadowing someone faster and better than me with the express purpose of improving my speed and general process.
At age 14, I was introduced to computers by a friend of my sister's. My sister was in college and I was visiting her. The friend was surprised at how much I was able to grasp given that I had no computer background (this was in the dark ages, when I had a yellow rotary phone and pet dinosaur).
So I think: A) Just letting him watch is perfectly fine. B) If he asks questions or has a specific purpose, you can tailor it some at that time.
I learned to code when someone hand-coded a small site for me and then gifted me the code. My then husband was double majoring in history and computer science. He had to explain FTP program to me and some other very basic things. Then I began playing with the code by just substituting color codes for the ones already there and seeing what it did. This is not very different from my school experience with French where they would give you a sentence ("The house is red") and have you substitute the one descriptor ("The house is blue", "The house is yellow").
Perhaps he will be bored. Or perhaps this will be a unique learning opportunity for him. Helen Keller learned to sign in spite of being both deaf and blind and a small child. Her parents already had some homemade "signs" for her to use to engage in crude communication and the teacher was just persistent in signing things into her hand over and over and over until, one day, she realized the signs had meaning. It just suddenly clicked. Then she ran around asking, in essence, "What's this?!" and "What's this?!" about everything she could get her hands on.
We don't fully understand how that happens, but it does.
FWIW: I homeschooled my two gifted-learning disabled sons for many years. My oldest thinks in pictures and needs broad context before the details make sense. Other people need all the details to build the broad context. Different strokes for different folks.