Why would my partner lose his hard on during intercourse?

With the information you provided, I will attempt to give you an equally in depth answer.

This could have happened for any of a hundred reasons ranging from he's tired all the way up to medical problems.

Hope this helps.

How old is he, how often does it happen ( it happens to everyone sometimes)
What is his health, what medication does he use.

how old is he, how often does it happen ( it happens to everyone sometimes)
What is his health, what medication does he use.

As well as his stress/exhaustion levels. Substance/alcohol/tobacco use?