Looks like a green spotted puffer to me. He will be ok in freshwater for a short time, but will eventually need at least a 29g tank with brackish water, heavy filtration and no tankmates. These fish are very aggressive and get more aggressive as they age. If you want to keep him, get the bigger tank as soon as you can, set it up as very low end brackish (SG 1.003 or so) and begin cycling it. After the fish is moved, you can slowly increase the salinity of the water to moderate to high brackish conditions (SG 1.015 or so would be good).
However, if you were looking to get an actual dwarf puffer, I would return the fish and be sure you're getting a dwarf puffer rather than a green spotted or figure eight puffer, both of which are fairly common and are sometimes sold as dwarf puffers.
4 8 15 16 23 42