Slurpees! My DD lived on them when she had her tonsils out in December. (She was 9.) Shakes and ice cream felt too thick and gloppy for her.
Do keep pushing the fluids. DD's recovery was very smooth, but it took her several days to be anything like herself again. I think she was supposed to drink 80 oz per day. We got two Super Gulp slurpees into her each day, plus anything else we could. Pain meds need to be non-negotiable. He won't feel well enough to drink otherwise. If it's the codeine syrup, put it in the freezer. It won't freeze, but the taste will be diminshed somewhat. We couldn't find anything to mask the taste, so DD just chugged it with a slurpee chaser. We moved to Tylenol meltaways asap. Try jello, broth (sometimes warmth is soothing), sherbet, ice chips, popsicles, anything to get fluids in!
Our neighbor's son had his tonsils out a little before dd. He couldn't/wouldn't swallow and didn't get much in the way of liquids in. He had a very rough time of it. His wounds became actual scabs and he had pain, bleeding, chunks of them coming up when he coughed, ugh. Scared the you-know-what out of me, so I made sure to keep dd hydrated when hers came out. (That's one of the purposes, to keep the scabs very soft and moist, so bits just come off as they swallow.) Sorry to be gross, but it was an eye-opener.
Good luck! Hope he's feeling all better soon!