only a month old? check your parameters!!! The cycle might not be over yet! And if it is not, puffers are not strong enough to survive in a high amonia /nitrite environment. Take a sample of your water to any fish store (I've had pleasant experience at King Ed, JL).
If everything is good - try silversides. Puffers usually love this stuff. Then you can slowly move him onto mysis shrimps....
Cheers!
PS. 70g is not bad for probably a year or so - depending on how much you feed it
