Cheerios dissolve into mush very quickly, so it's not too surprising that he'd be okay with those and not other things.
By 14 months, I'd want to have things checked out. I daresay it's just a normal toddler preference thing, but if there is something going on you'll want to get him help with it ASAP. Again not saying that only liking purees at 14 months is definitely a problem, it's just something that I wouldn't be comfortable with if it were my child.
Going through phases of eating all of one type of food is normal and okay. A study was done back in the 30's (sorry I don't have the citation ready to hand) where over several months they tested the blood of babies allowed to pick their own foods (all purees) and even though they'd go through phases of eating only one thing they didn't have deficiencies. Basically, the babies all ate what they needed and the researchers just made sure a variety was available.