I would suggest getting a new one, just get one to replace whatever type of hide it was (humid, cool,warm) I'd just get one that will be a good size for him even as an adult.
Ive seen people using lids to like butter or something for the floor of the coconut hide. If the inside is large and just the hole is small, you could use a dremel tool with a sanding type bit to make it larger just make sure there are no sharp edges. One of the hides I got for Amara it looks like rock but its made of some kind of fiberglass. Even for as little as she is still I had to make the opening bigger and it still looks natural. It also seems to be her favorite one too. Good luck!