I have a 9 month old lab.At present he is not eating as he should be,we give him a cup and a half in the morning and again in the evening.At the moment he's only eating once a day and when he does he doesn't always finish.We're having a pretty hot summer and I was wondering if it may have an impact.

That could be a factor. For a discussion of eating problems, see https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171
In the sticky at the top of the Dog forum.
A more tempting chow is seldom the answer.