We use Maximus for this (and just about everything else come to think of it).
It's really thick and lasts for ages.
Just a note of caution - make sure you direct the action and not your OH. You know best when you're 'full' so to speak. We've been working up to it for about 6 months now, and managed it fully just before Christmas.
We started by buying this: http://www.lovehoney.co.uk/product.cfm?p=34871 to get my OH used to somthing a little girthier than me or her usual toys. Our advice is (in working-up-to-it order!):
1 - Add a extra finger to the first attempt. I.e. If he normally uses two, use three.
2 - Keep using the same amount of fingers until you can take them all comfortably with no fuss, then add another. Repeat until you can have four fingers.
3 - Add his thumb.
4 - Get used to this until your very comfortable.
5 - Each time he has all five digits inside you, get him to put a little extra pressure on. Don't do this repeatedly and/or vigorously! Just have all of them, and push gently. This will get you used to the sensations and allow to work out when you can take more or when to stop.
6 - Keep trying this over and over, each time a little (TINY) bit further. Eventually he may be able to get past the last knuckle on his thumb. That's the widest point, so when you've got this far, keep repeating until it's easier.
7 - Warn him to be prepared to be sucked in! When we first managed it fully, she pulled the rest of my hand in and neither of us were expecting it!
You will need a lot of lube for this, and you'll have to keep reapplying it - no matter how thick or long lasting it is.
You will need to be hornier than you've ever been before - the hornier you are, the more relaxed you'll be.
It may not be possible for you - I've had exes who wanted to try this, but for whatever reason it just wasn't going to happen. Expect little, and anything extra is a bonus.
This is how it happened for us anyway, but YMMV. Hope this helps!
Before I forget - if you do suceed... Tell him to be prepared for your massive cheshire cat like grin! My OH was grinning for days and felt incredibly proud that she could fit my whole hand inside her...  Never been sure what to make of that really!
Happy fisting!