I'm not sure lying to him is the best option. He might realise you lied and that would be worse.
I agree you need to focus on him, though. Probably go through sex therapy together and see what you can do. But I feel you should be honest, so that he knows what you're sacrificing. Otherwise you will resent him for something he won't even know.
As long as you make it clear that you are here for him and that you won't pursue that other relationship because of the consequences on your husband, I feel it should be fine... I mean, the more that other man means to you, the more giving him up for your husband means, too. And it's important right now that he sees how much you love him and care about him.
It's your decision of course, but I tend to be of the opinion that while the truth can hurt, lying has the potential to be much worse, either by backfiring (he realises you lied) or for other reasons (the lie is worse to him than the truth and you didn't realise that).
Good luck either way. Take things slowly. Get your husband to enjoy sex again, that's your priority right now I assume. But if you're honest with him, at least you'll have the freedom to be sad about losing someone you cared for, and your husband can comfort you.