As far as I can find Joe DiMaggio holds the record for most homeruns hit by a player in their first 2 seasons, with 75.  McGuire would have shattered that, but he had 3 HR's in 53ab's in 1986 before crushing 49 and 32 in '87 and '88.

As of 8/28 Ryan Braun has 67. Last year Braunie averaged a dinger every 13.26 ab's (451/34) . This year its 15.06 (497/33), so over his career he averages a round tripper every 14.15 at bats (948/67).

Now, there are 29 games left. So far the kid has averaged 4.03 ab's per game (948/235) which means he can expect 117 ab's before the season is over and based on his HR production per at bat so far (117/14.15) he would hit another 8 HR's giving him a grand total of 75, tying the great Joe D.