Frozen peas, defrost them (or cook and cool them) and smash up just the guts of it- feed only a tiny bit.
It's not harmful for short term treatment- as they are poor nutrition and empty calories they shouldn't be used on a regular feeding basis. I normally shy away from it, and refer to other methods such as fasting first.. and for true bloating I would use Epsom. But you can use a pea if you feel you need to, and if this case calls for it.
Daphnia is more ideal for them, but often times is harder to find.
Not surprising that he didn't eat it, as usually they don't go for straight veggies. It doesn't smell nor look like what they normally eat.
Holding food for 2 days is the first step, can change the food after and see if that helps any as well.
Does he just have a rounded belly, or are there other true bloating signs, such as tipping over, difficulty swimming, etc? Rounded bellies, especially after eating, is quite normal. It is possible you may be over feeding at once as well causing an extended stomach. How much do you feed and how often? Of what?