It on sale of unlisted shares (private ltd company)

My client is the founder of a 5 year old private limited company. He is now selling some of his shares (partial divestment) to an external investor. Kindly help answer following queries:

1. The amount in question is substantial. How much income tax would he have to pay ?

2. Is there any way of reducing the tax burden in a legal manner ?

3. He plans to invest some of this money in to another venture - can he claim exemption for this investment?

Thank you very much for your help.