[souffle]

Aww bless him. I'm so sorry that happened.
Which foot is it that is affected front or back? Sometimes with a front foot eating can be more difficult as they can't sit up and hold the food to nibble it. Even with a back foot balance can be harder so that may explain the not eating as much.
Check his store as well. He may be eating a normal amount however as he has a missing leg may not be transporting food to the store and leaving it in the bowl instead which would explain why the bowl seems full. Emptying the pouches is harder on the missing foot side as the use both feet to push the food forward out the pouches.
He's likely still feeling sore and they can nibble at the wound so watch that closely. He may also be unsettled in a new larger cage. We normally use a smaller tank type cage after operations to restrict too much movement - a bit like a hospital cage.
Use lots of enrichment to keep him busy like cardboard tubes and boxes to trash and make puzzle toys for him to search food in. Hope he recovers soon