Dear Jane,
the highest amount of wildlife of all Kenyan parks / reserves definitely has the Mara. So this might be a very good choice for your son. BUT the wilderness experience sometimes gets heavily disturbed by the enormous number of tourists that frequent the reserve (same in Amboseli). Sometimes there seem to be as many minibuses as animals and many tend to take off, making new tracks wherever they feel it. The Mara triangle in the west of the reserve is a bit calmer, so I really recommend to go there.
I really like Samburu / Buffalo Springs! Many elephants, different wildlife (another giraffe and zebra species, famous for leopard sightings), beautiful landscape, great cultural experience when you get in contact with the Samburu tribesmen. Many people I spoke to really are overwhelmed of Samburu's increcible beauty when it got rain and is green. So this might also be a good option in the rainy season.
Tsavo has a fantastic wilderness feeling due to its size but wildlife apart from elephants is more difficult to spot (although it's definitely there) and some areas are also quite overrun due to the close proximity to the Indian Ocean resorts.
Lake Nakuru feels a bit like a big zoo. It's completely fenced and doesn't have a real wild atmosphere. However the sea of flamingos is overwhelming (when they are there what is not alweays the case) and definitely the best place in Kenya to see rhino.
I really love Mount Kenya which has fantastic scenery but probably not recommended to a trip with a ten year old boy...Due to the altitude you need time to acclimatize...
Generally I'd really recommend to concentrate just on two parks or reserves and to explore them intensively instead of racing from one place to the other. Many of the reserves are very diverse and have very different areas so it takes time to explore. Sometimes less is more !!!
However you decide enjoy Kenya and have a fantastic trip