I think it sounds like a great idea, Mao!
Since it’s not intended to be a “formal” training facility, and you’ll likely have dogs and people playing rather than training on the equipment, I’d keep things smaller scaled, safe and sturdy.
I have to admit that when a local dog park put up a full sized dog walk, I was pretty weirded out - I personally wouldn’t want my dogs anywhere near that in a situation as uncontrolled as a dog park can be - it really looked like an accident waiting to happen.
Maybe a low and wide A-frame - 2’-3’ high max, some PVC jumps and instead of a tunnel, some sunken tires that the dogs could go under/around/over if they’re big enough? A table would be good, too. I guess I’m thinking more “dog playground” than agility course.
I did a quick google for “dog playground equipment” and found a couple of links that might give your son some ideas:
http://www.puppyplayground.com/
http://www.korkat.com/content/dogparks.html
I’d also include a nice bench (for people and dogs) - for quiet bonding time, and some sort of poop pick up station - that could mean a designated spot for a rake and scoop or a bag dispenser with a trash can that’s easy to empty!
And if you do build agility equipment, I’d make certain to give it a safe surface - I’d suggest rubber. MAD Agility equipment sells a granulated rubber product that you press into paint when it’s wet and then paint over several times - it’s not as expensive as some of the poured rubber surfaces that you can build, and since you do it yourself, you could put the surface on anything you wanted (especially useful if you were going to custom make equipment!).