Recently our dog has started to wipe his bum on the grass after he has done a poo.
Is this his way of wiping his bum, or do I need to go get him checked out at the vets.
thanks

That sounds like his anal glands aren't being emptied. 

Is his poo nice and firm or has he been loose lately?  If the latter they may not be being expressed while passing a motinon which is what should happen. 

Some dogs simply need theirs emptying because they fill up too quickly. 

Your vet or groomer can show you how to do it, but it is a peg on nose job and use a good handful of tissue to catch the noxious liquid. 

The glands can get infected and impacted and if seriously so can cause a major problem.

I agree with Brainless it sounds like his anal glands aren't emptying.

warm regards susan