[souffle]

It's probably because he is using the sand as a potty. If he is only using that then the sand is not absorbent so the smell of wee is more obvious. You might be best to spot clean the sand daily and scoop out the wet sand. It may be the change of food but I have never found it does that myself. Older hammies often wee more so he could be getting it wet more rapidly hence you smell it.
If you think he could have a urine infection though then a vet visit would be in order for a check over.