TL:DR - it's a complex topic so buckle in for a longish read but a book is not necessary for Level 1.
NB: I have no qualifications but I have learned a lot for myself and my DCs. It's a long one, if you're willing to read.
These are all low stakes options (solid baby steps) but still vaild and will bring a good return whilst your DS builds his financial knowledge and risk tolerance. ISAs earn tax free interest in the UK but the interest/capital gain they earn is taxable in the US.
If the investor is a US person, not all of this applies and you/they should do a bit more reading online and elsewhere. It's not overly complex, just different. Be careful as a US person: your optionists are limited.
I'm not sure how to search for them but for UK citizens there are many, many threads on this across MN.
In the UK, there are some basic steps anyone can take to start growing wealth but there are slightly different paths depending on how you are earning. There are some specialist ISAs but you should research them for yourself (Jr ISA, Lifetime ISA and more)
If you are a UK citizen, the basics are:
open a stocks & shares ISA with a reputable bank/ buliding soc. and put in as much as you can, up to £20K per tax year (April to April) into it. Keep it there and add more if you can. If there are better S&S ISA deals, roll the original into a new one and add more funds if you can. Read about 'rolling' since it's a great way to put more and more into tax free growth.
Anything less than 20K frrom that first £20K that you put in the stocks & shares ISA, put into a cash ISA, perhaps for medium term since the best rates for these are usually in the first year.
If you have a workplace pension, (don't worry about having more than one pension - it's normal), put as much into that as you can whilst you are working there. It means you pay lower tax on your salary and your employer will contribute (match or better) and the govt. will too!! Keep ploughing as much into that as you can afford so that it will be waiting for you when you retire. Check whether you can contribute a one off just before the end of March.
If you don't have a workplace pension, open a SIPP, if you can, contribute as much as you can into that if you are working at a place with no pension scheme.
If you don't fancy the committment of an ISA, a cash bond will earn you good (but taxable) intrest in the short term - google. MoneySuperMarket will have some good bonds/accounts to buy also.