kluk as boyfriend is more or less only possible when you put a possession in front of it. MY, HIS, HER, MARY'S... Similar as muž for husband or žena for wife.
If you had a sentence MŮJ KLUK JE MALÝ = My boyfriend is little. Could mean my son, depending, but with no other context, boyfriend is more likely. But just "malý kluk" has no indication of a boyfriend and would be a boy.
young is mladý - a different word
Little and small mean the same for me, even if you sometimes prefef one or the other. In Czech it is just one word - malý.
Of course there are more words with a similar meaning, but they are slightly different (drobný, prťavý, malinkatý, maličký...).