Your son has impeccable taste.
It's always nice to hear about someone discovering Nadesico, considering it's generally considered to be some obscure relic from the distant past nowadays. It was fairly well received when it first came out, (I wasn't even interested in the show back then, but I was certainly aware of it) but hardly anyone remembers it now, much less talk about it. That's usually the case with most older shows anyway.
You're right about Gekiganger - it was hearing about the OVA that convinced me to give Nadesico a chance. Do you have the ADV release or the Nozomi collection? If you've got the later one, your son can check out the OVA since it's included in that set. You might want to hold off on showing him the movie though.
I've a lot of stuff in the backlog I haven't seen yet, not to mention all the partially-completed shows I need to finish. Recent additions include Voltes V, Giant Robo and now SSSS Gridman has joined the pile. Right now I'm jumping from one thing to another - recently finished a rewatch of Bubblegum Crisis, I've also started the original series of Sorcerous Stabber Orphen and I'm watching it alongside the new series that's airing now. I'm also just about done with the first season of A Certain Magical Index as well.