hi n2csb!
congratulations on deciding to get a puffer, I am a little concerned that you class yourself as a newbie as puffers arent the most hardy fish and the mbu will need a lot of care and attention.
That said if you do your research and make sure you do all you can for your puffer I'm sure you will have great success with him. I have also recently bought a mbu puffer (just over a month ago), my fifth puffer, put my first mbu. I have been talking to Clare, our resident mbu expert, she has a full grown mbu called Hamish so she knows her stuff.
Please check all your water parameters and post them. Did your lfs test your water or do you have your own test kits? (if you dont, go get some!!! you will need them)
as puffer_freak explained, any nitrites or ammonia in your water will definitely explain this stressed behaviour. Also my mbu has never gotten a dark belly, even the time he puffed up which must have been very very stressful for him, so dont take having a white belly as a good sign necessarily.
How much have you been feeding and are you syphoning out any left over food as soon as your puffer has finished eating? Rotting food wont be doing your water quality any good at all.
How long has the tank been cycled? That is up and running with fish living in it and an established bacteria colony?
are there any other fish in with your mbu? Have you tried doing a water change, adding stress coat, stress zyme, tea tree oil or melafix to the water?
if you do know your water parameters, what are they? (ammonia, nitrite, nitrate, ph)
pm me if you like, and I'm sure Clare will help you out with anything you need to know