If you put it in his name:
you have to consider the deeming provisions of s7 of the income tax act and corresponding sections in the 8th schedule for capital gains. Basically these sections say any income or capital gains that accrue to a minor child (<18 years old) from an event/asset/transaction whose ultimate funding was a donation from his parent (or a donation from another person to whom the parent also donated money (i.e. "indirect")) will accrue to the parent of the minor child
Essentially if you put R500 a month in Satrix or whatever this will mean you will have to include the dividends in your taxable income (not a huge deal as dividends are exempt anyways. I'm not sure if Satrix would accrue any interest (due to property stocks included within it) - again not a huge deal as you get R22800 interest exempt per year.)
If you sell the investment before he is 18 - you will include the capital gain in your income + pay tax.
Once he is 18 the minor child deeming provisions cease and he will be responsible for any tax consequences - nice as until he's working he can sell around R200k capital gains tax free (in todays money) until he hits the tax threshold (assuming no other income)
Also remember you are allowed to donate R100k per tax year tax free (100k cumulatively to anyone). I doubt you would be contributing more than that so it shouldnt be an issue.
If you do the investment in your name
There are no tax worries for 18 years - yay! (bear in mind tax changes a lot so what is the case now may be totally different in 10 years time)
When he is 18 and you want to give him the investment - you will have to donate it. A donation will cause a deemed disposal for CGT for you - so even thoough youre not selling the investment you would have to include the capital gain in your taxable income then (and be taxed @40% or whatever.) Then donate this to your son then - anything in excess of R100k (the current limit) will be taxed @ 20% in addition. Your son would then have the shares at a cost at the value then (not the original date) + the donations tax paid. If he were to immediately sell it he would make a 0 CG and pay no CGT
OVERALL
If you put it in your name there is less admin each year, but at the end of 18 years you will have to pay tax (at least CGT and maybe donations tax). If you put it in his name there is more admin, but at the end of 18 years you won't have to pay tax. You could also keep the investment in your name and pay him cash (no deemed CGT disposal - but then you have to pull a couple hundred k out of a hat. However, then you don't control this investment and then if he turns out to be a bastard child you can't use it as leverege

Also if you ever need the money - he will have to donate it back to you