No, it has to be a series! We want to lock him down for a few years at least. 
It's a good point, never really thought about him just doing a one off when he can.
But I think in the world of children's publishing, series are preferable, even expected. Expected by publishers and readers. It also might be expensive to negotiate book by book, so I'd think publishers would do everything they could to lock him into at least a 2 book deal.
I'm sure the publishers would want a long term committment; makes business sense, more money.
And the fans too - on the surface. However, I remember what the routine and grind did to good old John Gardner, the need to hit deadlines, produce something. We KNOW JG became jaded and the quality of his work faded as his enthusiasm waned.
Consider, Charlie has already written five Bond novels plus short story. I do not know how many years he has been involved nor the number of hours of mental engery committed to Bond, but I do recall his suggestion that he needed a break. Therefore, wouldn't an adhoc policy of novels when Charlie felt so inspired be better - to avoid the very staleness that befell Gardner, and, dare I say it, even Ian Fleming???
I, for one, would prefer to preserve the quality of Charlie's Young Bond with fewer but better rather than more, sure, but worse.
Less might be more, ultimately, you know.