I agree that you need to get advice from a lawyer (presumably US-based) who is well-versed in US immigration law. It could all go disastrously wrong if it is not done properly.

The point to emphasise, of course, is that although the employee will be physically in the US, he will still be employed on his South African contract and will be paid in South Africa for the duration.

As you say, the question of whether he might stay longer than the two and a half months is a different point, and probably oughtn't to be allowed to confuse the issue at this stage - although you do need to bear in mind that it might take a while to organise such a thing (i.e. the employee might well have to return to South Africa and return later).