What is the ruling of Islamic law regarding a husband who is excessively scrupulous about impurity (najasah), to the extent that he washes his legs if a drop of water falls near him, claiming that spray has splattered from it, and how can he be guided?

This waswasa (whispering/insinuation) is from Satan. To cure it, one must turn away from it and pay no attention to it, for indulging in it leads to its firmly rooting itself. Therefore, continue to advise your husband and warn him against indulging in these waswas. If he is falling short in his wife's rights, then he must fulfill them and treat her kindly, as Allah the Almighty says: "And live with them in kindness."