If I were you, I would forger about it until the weather takes a turn for the better, then he can go out in the garden with a bare bottom, or just some undies. He will then obviously become aware when he is wetting himeslf and if you keep an eye on him and rush the potty over at the right moment, he will twig on eventually.
My eldest son became 'potty trained' accidentally, when we were in Tenerife. We had a ground floor apartment with a really large terrace. The terrace had a small grid in the middle and a large plant pot in the corner. The little fella used to run around naked and then took great delight in weeing in the grid and seeing how high he could get it when weeing in the plant pot! We continued this by buying a potty for indoors and telling him to wee in that, as if it were the grid!
Children do, as a natural progression and as a human instinct. all eventually become potty trained or 'dry'...it's just that sometimes we want them to be faster than they are really ready to become...what with pre-school, playgroups etc. Don't get too anxious about it, as it will all fall into place eventually and then you'll be nappy free, which is a good day for any mother! x