we are planning a year traveling in Kenya, homebase Naivasha

Hello , very nice plans !!!
to my point of view you should get in touch with special lucrative associations here in Kenya , there is a lot that could show you the way , or start with your embassy to have it formal ,I know that German and Italian embassies are operating in this way (helping local communities ).
I will start to talk around me as I have heard about such volunteers programs .

regarding the long time car rental .... various options are available in Kenya , but 1st of all I guess that you are looking at a 4x4 car ?

let other expat revert to you but I will be pleased to support any further if I can

NB/ Naivasa is a very nice place and not very far from Nairobi , YOU WILL LOVE IT

Hello Boj and Helen, 
sounds like you're in for a treat, huh? Naivasha and Nakuru are just fabulous. I mean, my husband and I drive all the way from Nairobi to Naivasha on weekends just to have lunch and enjoy the place.
About long term car rentals there's a company called Mitech International Ltd in Nairobi that can sort you out. The company leases cars to so many expatriates here. They also have a tented camp in Naivasha (you can kindly check out their Website and Facebook page) you will also get their contacts, rates and all details online.
There's another company that leases 4×4 vehicles to expatriates, one of the company owners actually works for the US Army, Kenya. His number is ***. He can also tell you much more.
When you get here please get in touch, you must visit Lake Naivasha,  Enashipai Resort in Naivasha, it's just wow.
Your son will surely enjoy the flamingos on Lake Nakuru, best time is January to March. Mostly February.
I hope that info helps you.
Cheers!

Hello everyone and welcome onboard Rodeo1976,

Just to inform you that this new thread has been created from your posts on the Kenya forum for better interaction.

all the best,
Bhavna

Hello Samiol,

Thanks for your reply!

I visited 4 years ago Naivasha, stayed in Fishermans Camp, it was great.

Yes we are looking for a long term rental 4 x 4.

We will contact the dutch embassey for more info about volunteer programs. If you hear anything about projects in Naivasha/Nakuru?Gilgil?...............

Do you know anything about a visa run in Tanzania or other countries? We hear different things about this item. We want to stay 1 year in Kenya on a tourist visum.......

Best regards from Holland,

Boj

Thanks so much for your comments!

We'll stay in touch. I visited Naivasha with a friend of mine a few years ago, had a cottage at Fishermans Camp, so beautifull.

I will check out the leasing company and their site in Naivasha.

Best regards from Holland,

Boj

Hello ,
My name is peter mari (hakuna matata safari) i deal with car rental in Kenya and safaris am willing to assist you to plan your trip here .

Hi
I am Mostafa , from Egypt , i will be in Nairobi from 29 Dec. wish we can meet there guys . Actuallu i am life in Juja city and it's just 40 minutes from Nairobi