If one was never circumcised can they be called up to the Torah for their bar-mitzvah?
As with all questions of practical halachah, CYLOR (especially since there may be public policy issues involved). However:
Responsa Hillel Omer (Yoreh De'ah 144) addresses such a case. He says that the boy is certainly allowed to have an aliyah, considering that it's not his fault that he is uncircumcised; at that age the responsibility still rests on his father to arrange it.
In a different context, R. Moshe Feinstein (Igros Moshe, Orach Chaim 2:32:2) states that an uncircumcised kohen should still be called up first to the Torah, since after all he is a full-fledged Jew and kohen.
Naturally, considering that a bar mitzvah is supposed to be a celebration of the boy's responsibility for and acceptance of mitzvos, then if a bris can be arranged before the bar mitzvah - obviously, assuming that it is medically possible (that the boy isn't a hemophiliac or the like) - then so much the better; make it double celebration!