He did not fast Ramadan because of sickness 25 years ago and he has not made it up until now

Firstly: 

Putting off asking about the Islamic ruling on this matter for such a long time is clear heedlessness. Your husband should have asked about that immediately after he was bitten by that snake, especially since it was only one day before Ramadan. 

Your husband has to repent to Allah for this delay, and he has to regret it and resolve not to do such a thing again. And we ask Allah to accept his repentance. 

Secondly: 

Sickness is one of the excuses that make it permissible not to fast during Ramadan, based on the text of the Holy Qur’aan and the consensus of the scholars. 

Ibn Qudaamah said in al-Mughni (1/42-43): 

The scholars are unanimously agreed that it is permissible in general for sick people not to fast. The basis for that is the verse in which Allah says (interpretation of the meaning):

“But if any of you is ill or on a journey, the same number (should be made up) from other days”

[al-Baqarah 2:184]. 

This sickness that makes it permissible not to fast is severe sickness that will be made worse by fasting or there is the fear that fasting will delay recovery. End quote. 

If a person breaks the fast because of sickness, his case should be examined further. 

If it was a sickness for which there was no hope of healing or recovery, then he has to pay the fidyah (ransom), which is to feed one poor person for each day that he did not fast. Then the scholars differed if the person is poor and not well off – does he have to pay the fidyah if he becomes well off or is it waived in his case? 

But if there was the hope of healing and recovery from his sickness, then he should wait until he recovers and make up the days that he missed, and he does not have to pay the fidyah. Also it is not permissible for him to move from making up the fast to paying the fidyah instead. 

Al-Nawawi said in al-Majmoo‘, 6/261-262: 

If a person is sick and is unable to fast because of sickness that he hopes to recover from, he does not have to fast at present, but he has to make it up. This applies if he will face considerable difficulty in fasting. End quote. 

Ibn Qudaamah said in al-Mughni, 3/82: 

The sick person who has no hope of recovery may not fast, and he should feed one poor person for each day… this is understood to apply to the one who has no hope of becoming able to make up the fasts. If he does hope to become able to do so, then he does not have to offer the fidyah, and he has to delay making up the fasts and do that when he becomes able to do it, because Allah says (interpretation of the meaning):

“But if any of you is ill or on a journey, the same number (should be made up) from other days”

[al-Baqarah 2:184].

He only moves on to paying the fidyah instead if he loses hope of becoming able to make up the fasts. End quote. 

What seems to us to be the case — and Allaah knows best — is that what befell your husband was a temporary sickness from which he hoped to recover, and indeed Allah healed him. So he has to make up the days that he did not fast because of that sickness; it is not sufficient for him to feed the same number of poor people as the days he missed. 

But, if he feeds poor people as well as making up those days, this is on the safe side, especially since you say that he is now well off, praise be to Allah. 

See also the answer to question no. 26865 

And Allah knows best.