I'd give him a bath (luke warm water - just deep enough to cover his body) every 3 days for about 30 mins at a time - if he hasn't passed any hard stools (have a feel of them!) after about 4 baths, or if the bulge gets larger or if his behaviour changes, get him to the vet.
What has he been eating? Might be an idea to move down a size - smaller rodents will be easier to digest and less stress on his system.
What has he been eating? Might be an idea to move down a size - smaller rodents will be easier to digest and less stress on his system.