What is needed to be able to invest ?

A foreigner can invest in Kenya without application for more than a normal Single Entry Visa (visitors visa).  However, I notice that citizens of Sierra Leone don't require a visa to enter Kenya. 

Land and property probably remain the investment of choice, as income can be derived from renting and Kenyan real estate continues to steeply appreciate in value (house prices rose by an average of something like 16% last year alone).  Foreigners can purchase residential land plots and houses, but not agricultural land (unless the title is put in your name).  Returns on rent in Kenya are good.

He will need a PIN from Kenya Revenue Authority (KRA), to invest.  To obtain a PIN, one would normally renew your Single Entry Visa after 3 months and apply for a Foreigners Certificate (ID Card).  Once the ID card has been issued, the PIN can be applied for online.  If a visa isn't required, I imagine that he can just register for a Foreigners Certificate - this must be done on/before 90 days of being in Kenya, but am not entirely sure.

If your husband wants to register a company and start a business, he can do this process once he has his PIN.  However, he would not be permitted to work within the company without a work permit.  One of the requirements for this is that he has a minimum of $100,000 cash at the bank, or in the form of investments in Kenya.  As you are Kenyan, the company can also be jointly owned and operated by you.  It would be easier than going through the application process for the investors work permit.

I would also just say, as a UK national married to a Kenyan for almost 20 years, the government are singularly unhelpful in terms of allowing me to apply for either permanent residency, or dual citizenship, despite a lot of investment in Kenya.  I would recommend that your husband looks on this as an opportunity to make money, rather than assuring a stable future in Kenya.