You should collect oral as well as written evidence(s) to prove the amount (specific) has been given for 5 years for improvement of his business. The amount might have been transferred from bank (it is presumed) that is also one of the evidence(s).
Discuss with him (borrower) and settle the issue amicably otherwise there is limited scope (negligible chance) of recovery through the process of law.
Even the limited chances of success in a court of law is hit by limitation. You may try to settle the matter amicably. If not successful, you serve a Notice on him; further course of action depends on the response, if any.
Contact a local advocate and learn tricks of obtaining such evidence of his borrowing from you and if he is a businessman, try to obtain his financial statements with income tax returns as he might have shown his borrowing from you in his balance sheet (Which I believe amounts to an acknowledgment of debt) and payment of interest to you in his Profit and Loss account. Before all that just make inquiries with your auditor as to how you wish to disclose such sum advanced to him as your income for that financial year.
Given the fact that there is scant evidence, successful recovery is a long shot and even if you are successful in obtaining decree in your favour, by that time you would have spent equal, if not more, amount than you lent to your Cousin. And even then you will have a mere decree in your hands, virtually in the same position you are in ie. to recover the amount. To implement Decree you have to follow procedure through Court which involves more time and again expenditure.
Instead, take the assistance and involvement of family elders and strike a settlement where he admits all the debt, including interest and payment schedule and get it signed by your Cousin and elders duly getting it notarized. This time, if he fails, you will have very strong ground for legal action.
save all the call recording and bank transaction regards. limitations is continuous in these sort of cases. Give him a proper legal notice with criminal provision, most probably he ll settle on his own. Let me know if you need a detailed advices.