Just a quick update: after cleaning up his cage I may have found the culprit of Custard's weird smell. Lately he has been stashing food under his water bottle every night and by piling up the substrate has kept the bottle slowly but continuously leaking. The bedding under the bottle was really wet through and musty smelling and he digs in there every day for food. I feel so bad I had no idea it was getting wet
So have moved the bottle higher up so it can't leak.
Also gave him a new shoebox but he's mad cause hes been downgraded from Nike to Ugg