Aloha,
I don't think I know any experienced DM's or Instructors that would suggest diving Tunnels on your own until you have lots of experience and site knowledge. That being said, I don't know anything about your experience level etc. As a starter - there are only a couple places to properly enter and exit at the site without walking on the reef. If you have this knowledge you would be off to a good start. Tunnels is also a very large area and some people enter and exit further down the beach towards an area called Cannons. This wouldn't really be considered "Tunnels" in my humble opinion and it's a little different conversation. If you were entering in the traditional area (which has a very narrow channel to walk through to begin and end the dive) that is the area I can speak to best. The caution over the last 30 years I am personally familiar with is that if you enter in the proper area and want to go through the lava tubes, you must start the dive by going down current or where current "could be running." Generally speaking that is a diving "don't do." Following the course of the site you hug the walls weaving in and out of the tubes and channels to the west (your left as you stand on beach and look at ocean). If you go to the east when you enter the water it quickly gets very shallow and really isn't dive appropriate. It is very easy to not even realize that there is a current until heading back to the east. It is also not uncommon at all to start the dive with little to no current only to have it come up quite strong during your dive. If you can't make it back to the proper exit - that's when things can go quite bad. I believe there are still warning signs posted up there addressing recent rescues and last drownings. It's been a long time since I been up there, but these signs used to be common. You never have current running to the east on this site as the reef is enclosed to the east and the water spills into the site from that direction. This is what causes the flow of sometimes heavy current down to the west. Hopefully this is making sense. I probably should have had another cup of coffee (ha, ha). I'm really glad that it sounds like you caught it on a good day. There are many nice days up there for sure - and the site can be fantastic. I don't mean for this to sound like a scare tactic response, I'm just trying to be prudent and honest in letting you know that there are some unknowns to many people that dive / snorkel at Tunnels that sometimes lead to mishaps. I also don't believe there is a lifeguard at Tunnels. It great that the beaches that do have LG's that welcome questions about currents etc. to guide beachgoers. Especially with your son being new to diving and wanting to be sure he has an amazing experience you'd need to be really cautious in my humble opinion. If you were to come south you could dive Koloa Landing in the morning or pm and have much better critters in a shallow protected bay that is free of current. Just a thought.
There is a Kauai Scuba Club on facebook that you could look into to see if anyone might want to join you for a dive up there? I'd again just make sure it was someone who really knows the site and what they are doing. I did hear that a couple companies are trying to get their permits back. I don't know if or when this may happen. I hope this helps. Please let me know if I can help with any other info or explain things better. Happy dive planning.
Cheers
A
