What can I do to help my 7 wk old who is very constipated and colicy?

He is exclusively fed. 2-3 wks ago he stopped going every day. Knowing that's no big deal for newborns I tried not to worry. He became pretty hot one night so I went to take his temperature in his rectum(temp was fine) and out came poop and more poop, slightly more watery than peanut er. 5 days later and still no poop, I tried the thermometer again. Same thing as before, more poop only a bit firmer than before. Now it was more like peanut er, but with 'bits' that looked harder. About 4/5 hrs later, he had a normal watery poop so I thougth he must have gotten through whatever bothered him. Three days later with nothing, I took him to the docs (two days ago) who suggested 1oz water with 1tbsp caro syrup 3 times a day, thermometer once and see how he does. Well, he threw up the caro syrup in water so when i pump and give him bottles I am puting it in there. I tried the thermometer again around 4am and his stool is now a lot harder than peanut er.Help!