When to neuter?

My puppy is 5 months old and is starting to hump everything he can get his little paws on

When I rehomed him I signed a piece of paper to say I would have him neutered at 6 months old unless there was a medical reason not to, although I am a little worried as he had inguinal hernia op as a small pup, he was very very small and I don't want to effect his growth, and apparently it's better to leave Dalis a little longer to neuter because of their urinary stone problems.

Is it likely to effect him if I get him neutered at 6 months?

That sounds like a medical reason to me!