Compared to him getting a level every two kills, you'll probably have to kill a lot more and have a LONG and boring grind-fest ahead of you, just to get skills that you could have gotten already by now is what they're saying.
So yes, you could continue to level him but he's probably going to be not as great as everyone elses Donnel who changed him at 10/15. Not only that, but you're probably going to have to go out of your way to level him now. Also, don't call the people helping you noobs when you're the one who started the topic for help.
why should i have reclassed him at level 10? that's stupid. nobody who has ever played a fire emblem would ever do that. you reclass/promote your units once they reach the maximum level, so that way they get as many growths as possible.
unless... for some reason its impossible to reclass him once he reaches level 30?
So you basically called someone else stupid and when most people say he is right, you backtrack and say you want to things your way? Why bother asking the forums? Go do things your way.