Just to be kind of overly literal, a good Superboy analogue, with similar powers to the Sentry would be A) Kid Gladiator, B) America Chavez or C) Nova. Nova, IMO, would be the best choice, and probably look fine in blue and yellow (since his current outfit already is yellow and dark blue/black).
Others with kind of appropriate powers, like Thor, Hyperion, Blue Marvel, Carol Danvers, various Eternals (like Ikaris or Thena) or Hercules are already their own thing, and seem unlikely / less appropriate to be 'replacing the Sentry,' who is kind of a Johnny-come-lately compared to them (particularly the gods and prime Eternals), who've been around for centuries, if not millennia).
Rogue, as an 'ex-girlfriend' (regardless of how controversial that is) of the Sentry, might also make a suitable person to take up his mantle in his honor (at least, pre-married-to-Gambit Rogue...). As a plus, her flying brick powerset, stolen from Carol Danvers, is on-theme. Also, also, she looks good in anything.