I'm not a puffer guy, so this is just a general comment regarding very small fish in big tanks. This is one of the very few places where bigger isn't necessarily better, as the tiny fish may never locate the food. Keeping him in a smaller tank for awhile...the infamous "growout" idea...works but carries with it the decreased quality and stability of a smaller body of water.
I have on many occasions helped tiny fish get a good start by keeping them in a transparent plastic container or bin floating in the big tank. Drill a bunch of holes at one end of the bottom/sides, and then use a small pump or powerhead or even an airlift to direct a stream of tank water from the main tank into the other end of the bin. The fish is thus deriving all the benefits of a large water volume but is being confined to a smaller area making feeding easier.
Even when the small fish has tamed to the point where it eats food as soon as it is offered, this method prevents smaller torn-off chunks from washing away and becoming lost. Turning the flow down or completely off during feeding may be helpful.
The other way that has worked great for me is to situate a smaller tank right on top of the main tank, or on a shelf above it. Set it up so that an overflow from the small tank carries water to the main one, then use a pump to introduce water from the main into the small one. This won't be possible with an airlift, you'll need an actual pump for this.
Another alternative is to set up the small tank alongside the big one, so that the tops are level with each other. Set up two siphons joining the two tanks, then place an upward facing U on the bottom of one end of one of the siphons and turn it into an airlift. Again, the two tanks are linked, with water flowing only when the airlift is activated. Easy to change flow rate by adjusting air flow and/or by altering the height of the airlift. Depending upon what is more convenient, you can either pump the water into or out of the smaller tank, allowing the natural siphon to complete the other half of the circuit.
All of these ideas are fairly easy to cobble together, and can be made as permanent or as temporary as you wish.
