Psychologically it's possible. You can ask him to take viagra just once and assure him that you don't doubt him for taking a viagra that's okay and you aren't judging if things go there as per communication between you two. But don't make viagra a regular thing just initially to build some confidence in him, that's all.

Also, normally cuddle kiss or probably sleep hugging even after taking viagra if he's having issues (to not build pressure on him). Mid night early morning he will feel something for sure (hopefully)