How to crack that last bit of potty training?

[dodi1978]

DS is 2.9.

We are currently doing a mixture of big boy pants (or "boy boy pants", as he calls them) and pullups.
We know that he can recognise signs that his body can give him, because he occasionally announces that he needs a wee or even takes himself (although that only ever happens at nursery), and once or twice has even taken himself to the potty for a poo (last weekend, at home... although that was two minutes after showing my husband a reward I had bought in the supermarket for good potty behaviour... he must have twigged that, because here he was, pulling his trousers down and doing a big poo in the potty.... to then run for the reward with a dirty bottom...).
So, we know he can do it, but he is very on and off. Has really good days at nursery with barely an accident, and is actually quite good outside the house as well (one time he didn't go in the morning and we then went for an hour of shopping). He held it until we were back!
The problem is mainly at home where he just - bar bribery and rewards, and just a sticker won't do it - isn't bothered....
He is generally quite fascinated with poo at the moment. "No eat the poo" (ahem, he tried once or twice), pretends his big penguin has done a poo and requests baby wipes and nappies to sort him out...
I would now love for him to go for the finishing line as I have got my No2 due in early October and would love a few months of nappy-freeness! Any ideas?
We are going to see German grandma in late June, and I am tempted to say that Oma hasn't got pullups and nappies....
I am not bothered about nights just as yet, that can come later!

Any ideas welcome!