TTC, but husband can't perform - any advice?

I am trying to conceive and I am using ovulation tests. Yesterday the test showed an LH surge, so we have tried it last night and this morning, but he was unable to perform. I think he was under too much stress and trying too hard. The main problem is that he now feels like he has failed or that he is useless. What can I do? I have told him not to worry about it, that it is just the stress and that he'll be fine once he is able to relax a bit. But I'm not sure that it is helping. Should we try Viagra or something like that? Any advice would be appreciated!