Most absorbant booster?

Ditch the microfibre completely. It's like a sponge - absorbs quickly but wicks something rotten when compressed. You'd have much more success from something hemp, bamboo or zorb. If you're after larger sized inserts/boosters, a custom option might be your best bet. I think Nappies by Minki or Little Gumnut might be able to help out there. You could always weigh the nappy and bedding (before and after) to see how much pee there really is and how much absorbency you need.

I am pretty sure my partner will think I have but will try doing the weighing tonight.

I have never seen boosters advertised saying how much liquid they hold though so once I know how much he wee's how would I know which inserts / boosters would be suitable?

Once you know how much absorbency you need, you can experiment wetting your current boosters to see what they hold and what combination would work best. Water is close enough to pee for that purpose. It takes out some of the trial and error - and wet bedding!

In addition to what everyone has already said, I use a Hemp Babies " hemp big weed" (like a large prefold made of hemp fleece as part of our night nappy stuffing, it holds LOADS, I had it with a bamboo terry as well, but it was too much for us, but if you don't go for custom nappies, you might want to search out some? Might need 2? My son is almost 4 and wees loads at night. Dries quickly as well, which is a bonus. We have that, a large MF insert and a zorb booster and that usually does us (though...we sometimes have a leak...but rarely)

L.