Always better to feed outside the viv if you can, Like Saddleninja says... if they swallow any substrate it can compact them and they can get ill. Plus he won't get you mixed up with feeding time when your hand goes in the viv I feed all mine in a large plastic cat litter tray - it's easy to clean. Don't have any cats never have but we bought it specially for the snakes.
Thanks everyone. I have been feeding him in a separate large bowl It's just that I've got a new viv arriving next week for him and was considering using substrate in there instead of paper towel. Actually the cat litter tray is a good idea too and they don't cost much....thanks for the tip Lewy
Jasper has always been on wood chip substrate with no problems.
Another reason for feeding snakes outside the viv is that they will not then associate your hand entering their living area as feeding time. If you regularly put food into the viv they are more likely to go for your hand thinking it's food!
I use Aubiose which is a hemp based horse bedding. It is about £7 for a HUGE bale. I have 61 snakes and a bale lasts me weeks. It is dust free (and mite lol) and my snakes love it. I put mine onto it once they are a few months old but always feed in tubs in case of impaction from ingrstion