My boyfriend has frequent urination vomiting weakness no appitite and constant dry mouth. What can I do to make him feel better?

Dehydration. The best that you can do is take him to the doctor as soon as possible, sounds likely that he is dehydrated for the vomiting so besides to find out why he is vomiting he may require liquids thru the vein if he can not tolerate the liquids by mouth.
Take him to a doctor. He could have diabetes, high calcium, kidney failure, to name a few causes. He needs to see a doctor asap.