@Piu I'm afraid this is a classic scam. Every detail of his story is common to Nigerian romance scams, right down to the 6-year-old son. At this stage there is no way you can get your money back but please cut all ties with this criminal and never send a stranger on the internet money again. Nothing this man has told you about himself is true. He doesn't exist. He will just continue to feed you increasingly crazy stories - you will never meet him or get anything real or true from him.
Hi there. Looks like South Africans can get a visa on arrival in Uganda for USD 100, provided they can show proof of sufficient funds for their stay and an onward or return ticket out of the country. They will probably also ask that you provide an address and telephone number you can be reached at when in Uganda (hotel, organisation, relative etc). It is recommended that passports be valid six months beyond arrival date in Uganda.