nox, I had a very similar situation with my DD at a few months younger than your DS. She was previously happy with 3 bfs a day (morning, nap and night) and then suddenly started waking during the night wanting bfs. I thought it would pass, but after a while I couldn't handle having to go to work the next day when I'd been deprived of my sleep!

So I got a bit *tough* with her: no bbs during the night, bbs are asleep. I would offer her a drink of water and she was still able to come into bed and cuddle with us to help settle her. I took the sides off her cot so she could come in of her own free will too. It took a few nights, and at first she wasn't happy about it, but she soon learnt that if Mummy said it was still night and bbs were asleep, it was water and cuddles instead.

You might also try fitting in an extra bf in the evening, but since it hasn't worked with the cows milk, I'd say he's using bfing for comfort more than anything else.