Restless drinking

Hi,
We have a son who is just over 6 weeks old. However, he drinks his bottles very restlessly, kicking and hitting. It's quite a battle to get him to drink. But this isn't the case with every bottle. Last Thursday, I took him to the GP, who said he has something white in his mouth and suspected thrush. We got medicine for that, but so far, no improvement at all. Now, my son is quite congested at the moment and seems to have a sore throat (when he coughs, his face scrunches up and he cries a bit). Does anyone have any tips?