Permanent resident visa

Your husband would be able to apply for citizenship after you have been married for 7 years. Try the website below for information on this.
https://fns.immigration.go.ke/infopack/marriage/

Also check this site below for other visas or email them for information.
  www.immigration.go.ke

If you are working in Kenya, then he should be able to obtain a Dependents Pass.  If he has an income from South Africa (from investment, property rental and so on) equivalent to $24000 per annum, then he can apply for a Class K Permit.  The Dependents Pass is by far the cheapest option, though neither permit paid work.

After 3 years of marriage, he can apply for Permanent Residency.

A bit pointless e-mailling immigration, you are unlikely to get a reply!

For the permanent residency, you can apply now online, upload all the documents required sit and wait forever.

Unfortunately, immigration office in Kenya doesn't really work very well (if at all)

For Permanent Residency the application process takes around a year and a half.  There are some contributors on this site who have successfully applied.

As for dual citizenship...........those the applications that you would have a loooooong wait for!