Timur Shatatland's answer mentions "construction kits for kids". I like most of his answer, but disagree a bit with this point. At least if we are speaking about construction kits just aim at building one specific thing out of pre-made parts. Projects like on the photos you showed us will train his planing, understanding, selfreliance, skills and improvisation.
However I do agree that a set of tools can be quite helpful. I would also add some raw-materials. Focus on things that are rather cheap - knowing how to use those well will save you (and, later, your son) a ton of money.
Instructions and danger
You will need to instruct him in the save use of some of the tools, especially knives. There will be a few minor injures along the way, but 5-year olds can be tought to handle knives and hot things in a save way. For the first years you might want him to use those tools only when you are present, though.
Make sure you researched the dangers of a given tool before giving instructions: a power-drill, for example might be used by most adults, but many did never think about what happens when long hair or a scarf gets cought in it ;-)
Concrete list of tools and things:
Basic indoor paper-work:
- A stack of printer-paper.
- A box of mixed pens (should have a few colors).
- Leftover cardbord.
- scissors (different kinds are always helpful)
- Acrylic colors: Can be as cheap as 1Euro/Dollar per piece. Do buy black, white, red, yelow, blue and perhaps green. No fancy colors needed, since kid can learn about mixing colors (this can be a project on its own).
- A few brushes (the cheap ones made of plastic work great for me, while the cheap ones that look like wood and real hair usually loose hairs while painting, which is totally frustrating).
- Some stiff stuff to paint on (cardboard, old wooden things, styrofoam, ect.)
- Some old newspaper & instructions on how to avoid staining the carpet ;-)
Do show how to clean brushes imediatly after use.
Building stuff indoors (like floating ships and the like):
- 2 of those retractable carpet knives (one big, one thin). Do teach how to use those!
- Leftover styrofoam
- a mixture of small sticks and wooden leftovers
- a mixture of medium-sized or big nails or skrews (a hammer and similar tools might not be necessary at first, but will become useful sooner or later)
- Wood-glue (relatively inexpensive when bought in a big bottle, relatively harmless in regards to toxins).
- leftover bits of fabric.
- perhaps a glue-gun.
Many parents are nervous about those, but those are great for kids:
Kids do not have to wait for hours until the glue dries (which is frustrating for them), the glue of a glue-gun is relatively non-toxic, it glues almost everything that can be found in nature. It is hot at the tip, and the kid will hurt itself sooner or later. But it will just be a harmless bit of pain and perhaps a red spot on the finger, which makes a great teaching moment before giving the more dangerous tools.
- Nails & Hammer,
- Tools for digging,
- A place where he is actually allowed do these things
- Instructions and constant supervision.
Where to go from there?
It really depends on the interests of your kid. I have seen seven-year old girls soldering stuff, but whatever it is, you might need to follow Timur Shtatland's advices regarding tutors and such :-)
Do also use youtube for inspiration on techniques: Watch videos of builders together and talk about how you could use their techniques at home.
How to teach?
Seems like you are doing well: keep your kid motivated by helping when needed, but not setting the standard to high or intervening too often.