The transaction can either be treated as gift or a loan. In case you give this money as gift to him, there will not be any tax implications for you as gift received from specified relatives including parent are tax free in the hands of the recipient. The clubbing provisions will also not apply as your son is adult. 

Alternatively, you can advance this money as loan to your son which then can be invested by him in FD and which shall be taxable in his hand. The loan can be interest free or interest bearing loan. Please note that in case you give an interest free loan and are already paying any interest on the money borrowed, advancing of interest free loan will not work as the assessing office may disallow your claim for proportionate interest paid relatable to the interest free loan. 

Please note that just by gifting the money to your son, which is exempt in his hand, you will lose your right over your money.

In case you do not intend to treat this transaction as gift or a loan, the income tax officer may club the interest income applying the provisions of Section 60 which covers the cases of transfer of income without transfer of the underlying asset.