Q. A Ger that converted recently (less than thirty days) and is in doubt if he said veten tal umatar or retzeh does he have to repeat? Does it matter if he was already davening and bentching for a long time before his conversion?

A. Mishna Berura (188: 16) maintains that if one is unsure if he mentioned retze or ya’ale veyavo in the amida or birchas hamazon he does repeat, since we assume that he recited as he usually does and omitted the essential additions. (The above does not follow the opinion of the Lechem Chamudos (Brochos ch. 1) that differentiates between tefila and birchas hamazon – Sha’arei Tzion ibid. 12)
Horav Shlomo Miller’s Shlit’a opinion is that a recent convert that has not yet davened thirty days can be lenient and not repeat, however if he did pray before conversion, he is treated like anyone else since “hergel” or becoming accustomed is a natural human habit trait and does not depend on whether he was already obligated on mitzvos. The same would apply to a recent bar mitzvah.
See Chashukey Chemed (Gittin 4a) for a similar rule on a recent ba’al teshuva.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a