Show him a different 'physical' way he can let you know. Then be calm, but consistant in showing him everytime.
The only thing I can think to compare it to is my daughters. When I'm talking to their daddy or someone else and they need to tell me something, they have been taught not to interrupt. They come and place a hand on me . . . on my shoulder, my leg, my side, just anywhere. This tells me that they have something to tell me. I acknowledge by placing my hand on theirs and patting it. That tells them I will get to them as soon as there is a break in conversation. It took a bit for my 3 year old to catch on - been doing it since she was 2. Sometimes she will still need to be reminded. If she bursts out when I'm talking I will try to grab her hand and place it on me, still listening to the person I'm talking too.
So, maybe something like that? When he pulls up your shirt or he tugs it down, you could take his hand and place it on your arm or even on your stomach OVER your shirt and tell him, "Show mommy you want to nurse like THIS." Making sure to be consistant until he 'gets' it and then being attentive to respond when he does 'get it' and uses it to communicate.