BUBO provides the following three national list species:-
Kenya - 1,118 species
Tanzania - 1,055 species
Uganda - 1,043 species
So perhaps a combined list for the three countries of 1,500 species?
The combined African list is 2,580 species. He has added 798 species in Africa and perhaps, he had seen 500 species on the African list before his arrival.
The question is how many of those combined 1,500 species will he have seen already. I would guess - and it is a total guess - that he will have seen 800 of the 1,500 species already?
All the best