Is it permissible for my husband to pray in his underwear, which he claims is unclean, even though he has no health condition preventing him from praying?

The husband must repent from abandoning prayer and observe it at its prescribed times. There is no excuse for anyone to abandon prayer as long as their intellect is sound. Regarding the uncleanliness of his underwear, if the impurity is urine, feces, or prostatic fluid, then only the impure area must be washed. If he is unable to remove it, let him pray according to his condition, fear Allah, and maintain his prayer.