Suggest Medicines For Headache

Hi, My Dad two months ago was diagnosed with stage four sinus cancer. Right after christmas he had a radical surgery to remove a large tumor in his sinus cavity. He has no clean margins so radition will be starting soon. My question is he just now has started with severe pain in his head and he has trouble taking strong pain meds due to their nausea side effects. He is also missing part of his pallet so he is being fed through feeding tube so anything he takes needs to be crushed and sent through that. Any suggestions on a good pain med possibly liquid? and with lowest side effects of nausea.