It is also possible that the superwetter night nappies will see you through the night without a change. I have not used an all hours but I do have a superwetter by the same company & Em (the lady who makes them)told me that the all hours hold a similar amount to the superwetter night nappies.
All hours rarely come up on classified but if you want to try, Sam has some here for £10 which is an amazing price.viewtopic.php?f=57&t=99637
TBH going on what you said, he needs a heavy wetter or superwetter night nappy and they are more expensive. Out of interest, what nappies do use for daytime & how do they preform for you? I have heard great things about the baby beehinds night nappy from women with heavy/super wetters. They might be a more affordable option for new nappies.
I keep on editing this post coz I remember more to say. I would seriously consider using wool over the night nappies if you don't already use wool. They allow the nappies to absorb more.