No, Derry will not get back to his seclusion.The brief association of Derry with Mr. Lamb boosted his self-confidence and helped him to respect his own self. The manner in which the old man made Derry realize the importance of his being self-dependent, of respecting himself and of holding on to hope helped Derry undergo a remarkable change. The new found self-esteem makes him tell his mother that his looks are not important. He starts believing in his strengths rather than his appearance.
It is not likely that the ***** of Mr. Lamb would take him back to his secluded life. This big change in him is definitely there to stay and would not be undone due to setbacks.