الجواب حامدا و مصلیا
The price of all the saleable good in his shop must be calculated, then 2.5% of it will be given as Zakat for the first year. Afterwards, Zakat for the second year will be given from however much of the calculated value remains. In this manner, the Zakat of the following years will also be paid, until 8 years of Zakat will have been given.
And Allah knows best