2 schools of thought (which depend on how you are fixed for flex).
#1, You could retire him... especially if you need the flex (unless he's a freedot... then you could retire if you just want the freedot to use for next season). Or....
#2, You could keep going but don't put anymore flex into him. Instead, see how good you can make him WITHOUT boosting and with the damage already done. Will he ever be a good dot? No... but you can learn a lot from doing stuff like this. I know I did. he probably won't get any offers from good teams... might even end up on CPU teams... but the thought here is to apply what you've learned so far about dot building and see what else you can learn. Everybody here (except for Dpride, of course

) has made build mistakes and have learned how to
repair from such mistakes when made. Also helps you learn when/where you've made those mistakes.
Again... it's about you and your flex. If you need it, then I'd retire. If you don't think you could learn anything more, then retire. If it's a freedot and you need that back, then retire. But if you think you could learn something by trying to "save" the build as much as possible... and you don't need the flex/freedot spot... might as well see what else you can learn. See how high you can get his EL up by the end of his build without boosting and with build corrections.