It is not clear from your question where the snake bit. Whether it is in the work spot or at his house.
If it is in the work spot, it will definitely come under the definition of an accident under sec. 2(8) of the ESI Act, 1948, and that employee is entitled to both the Temporary Disablement Benefit and Medical reimbursement. In case of death as a result of a snake bite, the dependents will receive the Dependant Benefit. For all this, the employer has to submit the Form 12 (old Form 16) to the concerned Branch Office. No contributory condition and no past service are required.
Even if you don't submit the declaration form now, it can be submitted along with the valid reasons under what circumstances you have not submitted the declaration form and obtained the temporary or permanent card.
On the other hand, if the snake bite is at his home or elsewhere (not in the work spot or office), he will get the reimbursement of the medical expenditure from the ESI Corporation on submission of the medical bills and claim form through his dispensary.
For getting the medical reimbursement under the super specialty treatment, the employee has to be in the insurable employment for 9 months or more, and he should have contributed for at least 78 days during the relevant contribution period corresponding to the benefit period. If he satisfies the above condition, he can get the medical reimbursement in case the snake bite is outside the employment.