ew, I'm sorry, I agree with Bizzy - first it's really a good idea to get rid of the poo every time, not only is it much harder to clean off when it's been sitting there a while but you run the risk of giving him nappy rash
As to your original question - our boy is almost 8 weeks old and now only pooing every second day, and we've just made the transition to MCNs rather than sposies overnight. For night time we've got him in a Kushies fitted nappy (from Nature Baby) with a couple of booster liners and that does the trick, last night he slept 8 hours with no leakage.
(Although personally I don't recommend the Kushies during the day, they're too bulky and I much prefer our Snazzipants All-In-Ones just in terms of ease of changing.)