How to sooth my son with HFMD? He can't take any solids anymore. Sometimes drinking water is also a problem.

How old is your son? Perhaps you can give him "treats" like ice cream, sherbet or fruit juice popsicles. Ice pops and ice chips also help. You can also try making breastmilk or formula milk popsicles. The frozen milk will soothe the pain in his mouth and this way, he will still get some much-needed nutrients in his system. Hope your son gets well soon!