I'm afraid it sounds like it's just one of those things. I had some small fuzzi's rather than the one-sizes which clara was tiny, and i had a week or two where they were leaking. Unfortunately it is common in small babies, and there isn't a huge amount you can do about it I don't think. I had a mixture of nappies, so just tried something else, and then went back to the leaking ones a week or two later. It's amazing how quickly the fit can change at that age.
The most reliable option for young ones would be a fitted and a wrap as there's two barriers to stop leaks. You obviously have to choose which option is best for you, but personally having tried both, i actually found I prefer putting a fitted and wrap on rather than having to take the inserts out each time i go to wash and then put the inserts back in after they've been washed.... I found that much more of a faff than just using a wrap when changing the kids.
Other people may have other advice for you, but my suggestions would be to either pick up some small fitted nappies & wraps off of the classifieds, then if your fuzzi's fit in a few weeks, you can probably re-sell them afterwards for a similar price. My other suggestion would be to try some different inserts - I think the fuzzi's come with microfibre inserts which let the wee out when compressed, as it would be in a baby who I assume is not yet mobile. You could try some bamboo or hemp (or both), they hold more wee and hold onto it when wet. Most people find that microfibre doesn't last their kids very long, so chances are when your son is a little older you'd need to look at different inserts anyway. Hemp inserts can be bought from Easy peasy on ebay (about £10 for 5 i think), they're nice and trim. For bamboo when clara was tiny I actually used my small itti trifold inserts in the pockets and they worked fabulously and are also nice and trim and not wide between the legs so don't usually make the nappy gape any more.
eta: crossed posts with louise!