Mental Capacity in Contract: A Dilemma

Rahul, a 25-year-old, was diagnosed with Alzheimer's disease 2 years ago. His parents, as his legal guardians, took a loan of ₹5 lakhs from a bank on his behalf and signed a contract. The loan had a repayment term of 5 years. However, Rahul has no recollection of signing the contract or taking the loan. Now, he wants to disown the loan. However, the bank is unwilling to cancel the loan due to the signed contract. What legal remedy can Rahul's parents use to get the loan cancelled, considering Rahul's diminished mental capacity at the time of signing the contract?