His average monthly income, according to you, is about 4.5 lakhs and he is showing only 0.25 lakhs salary as income. Income Tax returns support his figure.
Now a days, no court is believing in Income Tax returns and they infer that the real income is concealed and they take them as basis to give a guess work and come to a near about exact figure. Even then, courts are not permitted to guess about 4.5 lakhs of monthly income by just seeing his qualifications, unless court will be fed with some authoritative documents.
No one keeps currency just like that. He invests it somewhere. It might be realestate property, shares and dividents and various bank accounts and finally locker. He cannot afford to put his investment in real-estate on some other's names, which is called benami transaction. It has been prohibited from 1988 onwards. A person cannot purchase property in any one's name except his or his spouse's name. So property papers must be lying with him and their details are available with the Sub-Registrar's office. For finding out where his property lies and what are the shares he is holding, a detective's assistance is required.
As you said that you are in a position to hire detective, you do the same and get the figures of his income with supporting documents. The detective can get the documents showing his income from all the possible hospital sources (as a director as well as leading surgeon) through his own methods.
These documents can be proved in the court of law by following the proper procedure. To file the documents along with the affidavit of the detective and by bringing the detective in witness box you can prove these documents. On your own, it is very difficult for you, to prove his actual income and you have to necessarily take the assistance of detective.