Have you talked through with him what he needs to do to look after her, e.g. re nappy changes, food, favourite activities, what to do if she gets tired, what to do if she gets grumpy or upset? I'm wondering whether he'll be quite so keen once he realises exactly what is involved.
This - he will also need to take a pram or some sort of carrier for a 13 mo. A pram in muddy woods will be a total pain and some way to warm up food for lunch...
Honestly, I would bide your time and leave him to realise that it's actually a lot of work to look after a child who is a bit mobile but possibly can't walk yet, who needs v regular food and milk.
Has he said what time he wants to pick up at? I would simply say that DD will be ready to go after her breakfast finishes at 7.30 and will need to be home by 4pm at the latest [to give you some time to wear her out if she has slept all the way back. ]
Then I would email him a very detailed copy of her daily routine, instructions to buy an all weather suit and wellies, and a carrier and leave him to make his own mind up. 
I know it must be really hard to watch someone want to go off with your most precious possession. You don't say that he has been a poor parent to his daughter though? If anything were to happen to you, even temporarily like appendicitis, she would become his responsibility. It is in your daughters interests that she knows her Dad well.
BTW - weather forecast is rubbish for then 