Have a look at Windrush 14. WA design for single or 2 up, ideally adult and youth. Trapeze available but not compulsory. Fantastic fun and don't nosedive (easily). Mast is foam filled and helps not turtling. Parts still available and the newer fat heads are still competitive/ing. Can rig singlehanded (PM if you do get one). Make sure you get the kickup rudder version and preferably foam hulls.
Try Catsailer.net forum is a goldmine of info but not terribly active. I got mine for a song and have since regrettably sold it. Too many toys not enough time. Good luck

I think a Hobie would be too much boat for one adult and a 7 yr old. Agree with slammin that Windrush would be spot on. Enough thrills. Def. kick up rudder. I tore one off on a rock (the only rock?!) along Point Walter spit on a glorious eyelid peeling reach. Messy.
Eventually got tired of getting wet and bought a GP14 got tired of launching that and bought a 24 footer that lives on the water...
Might be a lousy time to buy. The market seems pretty quiet.

Hey I know someone that has my old Nacra 5.1 cat in Mandurah. I think he is selling it. It's not set up as a normal Nacra with a 900mm reef in the near new main and a super simple set up. Only hassle is getting the mast up though. PM if you're interested

Has your son ever sailed before MikeyG is the big question? It really depends on your childs personality and abilities on water. Strange things happen to kids...those you think would do well can be terrified and others are amazed...
Depends on your circumstances as well...what happens if your son is uninterested? Then you are left with the boat.
If you want to test the waters with your son and cats before buying, renting one of the Windrush 14s at Coode st, South Perth is perfect. They have jibs so you can really power up and on a good day get a hull flying if needed.
I know you have asked secifically about the H16 however the W14 mentioned is probably a safer option and fast enough to be fun...especially If he doesnt like sailing and you end up with the boat alone. Your son may be able to help a bit with sailing due to less tension on the boat which takes some pressure off yourself to sail.
The H16 is good fun...no doubt about it...however I would be planning on doing it all yourself. Rigging and sailing. Its a handful solo and you should have a reasonable amount of fitness to do so. Also with rigging it. About 2hrs to set up until you learn it well. I sail and rig solo and its doable for sure but over 20kts sailing it can be hard work and rigging is something you need to enjoy. Would not go out with kids over 20kts solo. I always sail safely with kids on board and would hate to flip in any conditions with a 7yr old onboard a H16. Seriously....
The H16 needs careful attention when powered up but is amazing when going full steam. Again personally would not take 7yr old out at full power due to risk of flipping. Consider the time to set one up if trailering always. The time it takes could also put your son off.
Never put a W14 rig up but looking at them I could see a 7 yr old being perhaps able to help out. Surely would be an easier quicker job too.
The Nacra is something to ponder as well. Never sailed one however I do like how they have no boom to worry about cracking your head on.
Whatever the choice remember to go at his pace as opposed to yours if you wish to sail with him for any length.

If you still want a H16 after reading this then happy to point out a few tips on what to look for when buying.

Buy a Windrush. Far more suitable for a father son combo. Much more user friendly than a Hobie 16. Hobie 16's are big and scary for even a pair of adults and have lots of bad habits, the worst being they are difficult to right. If your set on a 16 footer then look at Prindle 16s. These are far superior to Hobie 16s and don't suffer the sinking rear of the hulls when tacking that the Hobie does. Might want to consider a righting lever between the hulls to help right either of the 16s, father and son might not have enough weight.
Spend some time at a catamaran sailing club and check the numbers of cats in each class and get some sort of vibe on what suits.

Not making any personal recommendations around which boat to get but nothing too out there, I've sailed Hobies and Prindles plus a custom cat my Dad and I(pretty much "watching") built(missed out on a trial sail on the "Surfcat" 12(Windrush?) at Balmoral so my Dad did lots of looking and a couple weeks after Chrissy we had one in ply with 9 coats of varnish on coachwood ply to sail around Port Hacking. My next door neighbour when I was a kid had a progression of - Arrow- mozzie- Finn- NS & a few more, Bloke over the road was a Shipwright and built a few of them & used to restore clinker & carvel launches/ boats, Dad build a yacht in the yard & a few others, Hartley 16, Tasman Tiger, Vee Jays... the 70s was big on sailing, I'm a Shipwright & coming up on 40 years now & still love all of it.
Best thing is to make it about the whole off the beach fun lifestyle including kicking back under the sail with the boat laid over on the beach, do sandcastles, do picnic, do Mum along too, let the young ones take the helm early, sailing is an awesome way to build confidence and self reliance, be big on positive reinforcement but all within limits. Kids can and will get scared, get them to help with maintenance, kids love more than anything real time with their parents. Be ready to compromise and not go out... collect shells, make seaweed pictures, look in rockpools, go fishing, put them back, draw love hearts in the sand with all the family & don't care about sand in the car or rust in the roof. Canoes can be great too & calm waters are a great intro.

All the best from Jeff

I got myself an Hobie t2 just now. Quick set up. Sails easy.No boom to watch. And not heavy to move around on the beach.