If I've understood you correctly, the parents are broke, but they have managed (in the past) to fully pay off the mortgage. The son is afraid that if the parents get into financial trouble (which seems likely), then someone will lay a legitimate claim against the house, to get it sold to pay off the parents' debt. The son doesn't have enough money of his own to support the parents and save them from this plight, so he (and perhaps the parents) want to at least secure what they can by getting the house out of the parents' name(s) and into the son's name.
The son hopes that it would be okay for the parents just to donate him the house, and the only fees would be the transfer fees, to get it into his name. Several people have explained that it doesn't work like that because SARS will charge tax on the deal.
The better option seems to be for the son to actually buy the house from his parents. He can do this only if they really do own the house in their name(s). Do you mean, here, that the house is not, in fact, in the parent(s) names? If not, why not?
.... their names isnt on the deed!
If they do own it fully, with all the right paperwork, then he can buy it.
What others have written seems sensible. To set out speedster's explanation in a bit more detail:
The parents and the son decide, together, on the purchase price (let's just say 1'000'000). The son offers to buy the house. The parents accept his offer. Now he must pay them 1'000'000. He doesn't have that.
At the same time, in a separate arrangemnt, he asks the parents to loan him 1'000'000 (or a bit less). They say yes, and the contract states the interest rate, and that he will make repayments at the rate of 100'000 per annum.
On his next birthday, his parents give him a nice birthday present of 100'000. Or at least, they want to do so. After all, SARS has a view on gifts of this size, as "small enough".
The son says: "Wow, thanks! I know you don't actually have that cash, so can't we just subtract that from the amount I still owe you for the house?" The parents agree, and now he owes them only 900'000 (approximately, as the interest must be calculated).
If the parents do this every year, they will empower him to buy their house immediately, and gradually pay off the loan using the gift they give him, each year.