Baby really unsettled when drinking bottle (formula)

My son is 5 weeks old and has been really unsettled when drinking his bottle - outbursts or crying, arching back, stiffing up, pushing bottle away from face, looks like hes straining to poo but never is.

This only happens when drinking.

Weve been putting baby gaviscon in his milk for the last few days but no difference.

Anyone any ideas? Im going to call the doctors tomorrow.

I do use dr brown bottles which have nassivkey helped with wind and he brings it up easy so i dont think its wind.