R..
The sores in the mouth are the WORST my son would drool rather than swallow. Then they broke open and so he would drool bloody saliva, and cry.
They told me I could coat his mouth in milk of magnesia. Didn't seem to help.
What did work for him was using one of those mesh bags attached to a ring (I don't know if they make them anymore but it was for kids to try table foods without choking). I went to Sonic and bought a bag of small crunchy ice they sell and would fill the mesh bag with ice and let him suck on it. I would also place him in his high chair with the bag of ice and a bowl of ice filled water and he would dip the bag in the bowl and suck on it.
The cold really seemed to help his mouth and he didn't get dehydrated.
Good luck I think that was the longest week of my life!!