Hi.. my english staffy is normally bright and bubbly but yesterday and today he has just been moping around, sleeping and relaxing.
He's still eating, but not quite as much as normal. He's the same with water.
He's only vomited once yesterday and there's been no coughing or sneezing.
I have him booked into my vet in 48hrs so I'm wondering if anyone can give me any advice on what might be wrong.
I have read the blog about parvo which worries me a bit.

If he is up to date with his vaccinations parvo is unlikely (but still possible) in a 7 month old.
How are his stools? Any diarreah?

Thanks for getting back.. he's up to date with his vaccinations and doesn't have diarreah. He did just get back to the city from a week on a 50 acre block with another dog who he played with constantly. Could this be a factor?

It could have something to do with it, dogs do pine for their companions and he may have bonded really well with the other dog and is missing him.
One-off vomiting I generally ignore, but more than once or over a few days and you need to look at it.

Is there a chance he could have picked up a tick? They can often take 3-4 days to show any effect.

He could just be happy to be home. I take my dogs camping on the week ends where they basically have free roam. They run, swim, walk and play all day long. When we get home, they are just happy to relax in the comfort of their beds.