It is tricky with older lambs, unfortunately. Do you have other lambs you are bottle-feeding? They do copy each other, so if you can have it penned with other lamb(s) while you feed it/them, this one may get inquisitive and start trying to copy its mate(s)
I have one such at the moment. Strangebod has teatphobia I think, he's interested and wants to copy Snotter his mate (came in with pneumonia, all well now apart from the projectile snotting as he finishes his bottle - eeeyewwww) but as soon as the teat touches his lips he seems frightened and runs backwards. I am now feeding him with the teat concealed in my hand while I get it into his mouth. We finish each feed with a few moments of 'facing the fear' but progress is very slow...