I looked at spawning and reproduction of D. holocanthus today, and found a few possible things to take into consideration while attempting to make this work. (i should have written down the sources so you could obtain copies for yourself)
First off, reproduction in the aquarium has occured and is not impossible. In all of the literature that i looked over giving a time frame, spawning always occurs between 9:30 and 11:30 pm. this will be the time to check if your male is doing his job. The problem is that in the cases sited anywhere from 2 to 5 males will nudge the female to the water's surface and encourage her to release eggs where all of the males will frantically try to fertilize them. Not anywhere could i find information where a single male will attempt this. It is a possibility that males will become stimulated by the presence of other males to spawn and be sure it is their genes that are being passed on.
something you may want to try (if possible) is temporarily introducing another male into your system. (although this may be risky to the puffers you have now)
Another thing, spawning occurs when water temperatures are at or slightly above 24.4 degrees celsius. adujusting your water temp accordingly may help. The eggs are buoyant, and stay at or near the surface usually in or near floating sargassum. Temperature in the upper column seems like it would be higher in these areas, and the eggs may also be warmed by the sun, I havn't been able to find any information supporting this, merely one of my own speculations. (Something you might want to play around with.) Hatching occurs from 103 to 108 hours after being deposited. There has been no evidence of the eggs being guarded by the male or the female, so separating them may be a good idea. The larvae are shelled, and have no spines. until reaching roughly 16 cm. they will float in these sargassum clumps until development is complete and they will then settle into the reef.
An article that looked to be very informative (if you can find it in english rather than japanese) is:
Sakamoto, T. & Suzuki, K. (1978) Spawning behavior and early life history of the porcupine puffer, Diodon holocanthus, in aquaria. Japanese Journal of Icthyology, 24(4):261-270.
I am going to try to locate a translated version tomrrow. if you can, you may want to try to get a hold of this. (if i find it, i will make a copy and send it to you via e-mail)
I did not get a chance to make it into the library of Natural science today. i will look into it tomorrow and should get back to you on it before 9 p.m. hopefully i will find some more information there. They may have a copy of the periodical i mentioned. (hopefully in english) I will also check into the department of fisheries and wildlife. maybe they will have something usefull.
Until tomorrow, best of luck...