Is there anything I can place in his indoor plastic cage that will aid keeping his claws trimmed?Maybe sandpaper- or would that hurt him if he digests it?
I managed to make his toes bleed as he moved as I last cut them, so i'm eager not to cause him any more trauma than I have to in the future.
He's been kept inside all his life(almost 4) so I am not keen on putting him outside unless in the summer and in a run. But its unfair on him to cause him distress whilst cutting his claws.

Go to the vets and get the vet nurse to show you how to clip them safely. You need a decent pair of clippers as well, they can advise.

you could try giving him a small concrete paving slab or a brick in his cage to help wear them down, otherwise i'm afraid they will have to be clipped.
sandpaper would be a bit rough on the rest of his foot i'm afraid and he will likely just avoid going near it.

if you're really not comfortable clipping them yourself, then go to a vet.
it should only cost around £5 for a trim, but this varies wherever you go.

in the long run, the small amount of distress caused by clipping his claws every month or so, is far less than letting them grow too long and eventually affecting how he walks.

you can get quick stop powders which will stop the bleeding almost immediately, but unfortunately guineas moving when having their claws clipped happens
it's not the end of the world for them and it's never made any of mine any more wary of having them done in the future

I don't think there is much option except to clip them if they are just going to be indoors all the time.

What I found useful was to wrap them in a towel to stop them wriggling too much and then you have both hands free. Be careful are you found out they have a vein (or quill i think its called) that extents beyond the root. If you shine a light under the nail you can see where it ends.

mine live outdoors but still need the occasional trim. they seem to have gotten used to it now and I give them a treat afterwards.